Statements Of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Operating activities
Net income	$ 69,304	$ 57,223
Adjustments to reconcile net income to net cash provided by operating activities:
Net amortization of premium related to mortgage-backed securities	29,542	12,854
Amortization related to interest rate swap agreements	92	63
Share based compensation expense	429	207
Hedge ineffectiveness	169	145
Net gain on sale of mortgage-backed securities	(2,505)
Changes in operating assets and liabilities:
Increase in accrued interest receivable	(240)	(11,122)
Decrease (increase) in other assets	247	(227)
Decrease in accrued interest payable	(1,209)	(921)
(Decrease) increase in accounts payable and other liabilities	(333)	211
Net cash provided by operating activities	95,496	58,433
Investing activities
Purchases of mortgage-backed securities	(2,266,198)	(4,030,834)
Principal repayments of mortgage-backed securities	1,066,114	629,847
Sale of mortgage-backed securities	598,164
Net cash used in investing activities	(601,920)	(3,400,987)
Financing activities
Issuance of common stock, net of cost of issuance	562,183	746,716
Cash dividends paid	(69,141)	(46,116)
Increase in restricted cash on swap arrangements	(12,173)	(16,480)
Proceeds from repurchase agreements	46,000,699	30,797,598
Principal repayments on repurchase agreements	(45,606,444)	(27,982,909)
Net cash provided by financing activities	875,124	3,498,809
Net increase in cash and cash equivalents	368,700	156,255
Cash and cash equivalents, beginning of period	347,045	112,626
Cash and cash equivalents, end of period	715,745	268,881
Supplemental disclosure of cash flow information
Cash paid during the period for interest	42,318	27,115
Supplemental schedule of non-cash financing activities
Obligation to brokers for purchase of unsettled mortgage-backed securities	$ 4,226,273	$ 1,844,750