Statements Of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Operating activities
Net income	$ 284,373	$ 169,500	$ 174,400
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Net amortization of premium related to mortgage-backed securities	93,335	41,778	19,363
Amortization related to interest rate swap agreements	346	226	170
Share based compensation expense	1,150	1,432	1,296
Hedge ineffectiveness	655	210	(344)
Net gain on sale of mortgage-backed securities	(20,576)	(13,551)
Changes in operating assets and liabilities:
Increase in accrued interest receivable	(25,052)	(2,413)	(7,105)
Increase in other assets	(1,881)	(20,399)	(291)
Increase (decrease) accrued interest payable	1,419	208	(5,657)
Increase (decrease) in accounts payable and other liabilities	1,111	(721)	(47)
Net cash provided by operating activities	334,880	176,270	181,785
Investing activities
Purchases of mortgage-backed securities	(13,351,128)	(5,728,275)	(2,965,826)
Purchase of security, held to maturity		(5,000)	(10,000)
Principal repayments of mortgage-backed securities	3,559,612	2,467,288	1,190,153
Sale of mortgage-backed securities	1,766,557	563,688
Net cash used in investing activities	(8,024,959)	(2,702,299)	(1,785,673)
Financing activities
Issuance of common stock, net of cost of issuance	860,602	271,373	(223)
Cash dividends paid	(255,827)	(179,027)	(146,228)
(Increase) decrease in restricted cash on swap arrangements	(161,592)	(14,061)	5,366
Proceeds from repurchase agreements	163,847,236	76,875,143	63,174,301
Principal repayments on repurchase agreements	(156,365,921)	(74,540,601)	(61,347,217)
Net cash provided by financing activities	7,924,498	2,412,827	1,685,999
Net increase (decrease) in cash and cash equivalents	234,419	(113,202)	82,111
Cash and cash equivalents, beginning of period	112,626	225,828	143,717
Cash and cash equivalents, end of period	347,045	112,626	225,828
Supplemental disclosure of cash flow information
Cash paid during the period for interest	143,243	95,715	101,924
Supplemental schedule of non-cash financing activities
Obligation to brokers for purchase of unsettled mortgage-backed securities	$ 397,936	$ 840,157	$ 46,453