Our total operating expense as a percentage of our average stockholders' equity was 1.55% and 1.58% for the three months ended March 31, 2015 and 2014, respectively.
Dividends and Income Taxes
For the three months ended March 31, 2015, we had estimated taxable income available to common stockholders of $166 million (or $0.47 per common share), largely unchanged from $167 million (or $0.47 per common share) for the prior year period.
As a REIT, we are required to distribute annually 90% of our ordinary taxable income to maintain our status as a REIT and all of our taxable income to avoid federal and state corporate income taxes. We can treat dividends declared by September 15 and paid by December 31 as having been a distribution of our taxable income for our prior tax year ("spill-back provision"). Income as determined under GAAP differs from income as determined under tax rules because of both temporary and permanent differences in income and expense recognition. The primary differences are (i) unrealized gains and losses associated with interest rate swaps and other derivatives and securities marked-to-market in current income for GAAP purposes, but excluded from taxable income until realized or settled, (ii) timing differences, both temporary and potentially permanent, in the recognition of certain realized gains and losses and (iii) temporary differences related to the amortization of net premiums paid on investments. Furthermore, our estimated taxable income is subject to potential adjustments up to the time of filing our appropriate tax returns, which occurs after the end of our fiscal year.
The following is a reconciliation of our GAAP net income to our estimated taxable income for the three months ended March 31, 2015 and 2014 (dollars in millions):
Three Months Ended March 31,
Estimated book to tax differences:
Premium amortization, net
Realized (gains) losses, net
Capital loss carryforward
Unrealized losses, net
Total book to tax differences
Estimated REIT taxable income
Dividend on preferred stock
Estimated REIT taxable income available to common stockholders
Weighted average number of common shares outstanding - basic and diluted
Estimated REIT taxable income per common share - basic and diluted
Beginning cumulative non-deductible capital losses
Utilization of capital loss carryforward
Ending cumulative non-deductible capital losses
Ending cumulative non-deductible capital losses per common share
Our estimated taxable income for the three months ended March 31, 2015 and 2014 excludes $115 million and $102 million, respectively, of estimated net capital gains, which were applied against our prior year net capital loss carryforward. As of March 31, 2015, we had $648 million (or $1.84 per common share) of remaining net capital loss carryforwards, which can be carried forward and applied against future net capital gains through fiscal year 2018.