             Impact of Changes in the Principal Elements of Economic Interest Expense  Periods ended June 30, 2018 vs. June 30, 2017     Due to Change in Average   Total Increase / (Decrease)   Borrowing / Swap Balance   Borrowing / Swap Rate  Three months ended:       Repurchase agreements and other debt interest expense  $  125 
  $  25 
  $  100 
 TBA dollar roll income  implied interest expense  37 
  — 
  37 
 Periodic interest rate swap costs  (76  )   6 
  (82  )  Total change in economic interest expense  $  86 
  $  31 
  $  55 
       Six months ended:       Repurchase agreements and other debt interest expense  $  233 
  $  52 
  $  181 
 TBA dollar roll income  implied interest expense  76 
  4 
  72 
 Periodic interest rate swap costs  (112  )   13 
  (125  )  Total change in economic interest expense  $  197 
  $  69 
  $  128 

The increase in our average borrowing cost over prior year periods was a function of a higher federal funds rate, which was partly offset by a larger portion of our Agency repo funded through our captive brokerdealer subsidiary, Bethesda Securities, LLC ("BES"). On average, repo sourced through BES carries a lower cost of funds than comparable traditional bilateral repo. The decrease in our periodic swap costs was primarily due to an increase in threemonth LIBOR received on our payfixed receivefloating interest rate swaps. The table below presents a summary of the ratio of our average interest rates swaps outstanding, excluding forward starting swaps, to our average mortgage borrowings for the three and six months ended June 30, 2018 and 2017 (dollars in millions):
                    Three Months Ended June 30,   Six Months Ended June 30,  Average Ratio of Interest Rate Swaps (Excluding Forward Starting Swaps) to Mortgage Borrowings Outstanding   2018   2017   2018   2017  Average Agency repo and other debt outstanding   $  47,823 
  $  38,945 
  $  48,690 
  $  39,073 
 Average net TBA portfolio outstanding  at cost   $  16,912 
  $  16,931 
  $  16,252 
  $  15,205 
 Average mortgage borrowings outstanding   $  64,735 
  $  55,876 
  $  64,942 
  $  54,278 
 Average notional amount of interest rate swaps outstanding (excluding forward starting swaps)   $  42,804 
  $  36,549 
  $  41,918 
  $  36,161 
 Ratio of average interest rate swaps to mortgage borrowings outstanding   66  %   65  %   65  %   67  %           Average interest rate swap payfixed rate (excluding forward starting swaps)   1.79  %   1.52  %   1.76  %   1.50  %  Average interest rate swap receivefloating rate   (2.17  )%   (1.14  )%   (1.91  )%   (1.05  )%  Average interest rate swap net pay/(receive) rate   (0.38  )%   0.38  %   (0.15  )%   0.45  % 
For the three and six months ended June 30, 2018, we had an average forward starting swap balance of $3.9 billion and $3.6 billion, respectively. For the three and six months ended June 30, 2017 we had an average forward starting swap balance of $2.5 billion and $1.5 billion, respectively. Forward starting interest rate swaps do not impact our economic interest expense and aggregate cost of funds until they commence accruing net interest settlements on their forward start dates. Including forward starting swaps, our average ratio of interest rate swaps outstanding to our average mortgage borrowings for the three and six months ended June 30, 2018 was 72% and 70%, respectively, compared to 70% and 69%, respectively, for the prior year period. Net Interest Spread The following table presents a summary of our net interest spread (including the impact of TBA dollar roll income, interest rate swaps and excluding "catchup" premium amortization) for the three and six months ended June 30, 2018 and 2017:
