Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating activities
Net income	$ 349,245	$ 284,373	$ 169,500
Adjustments to reconcile net income to net cash (used in) provided by operating activities:
Net amortization of premium related to mortgage-backed securities	162,844	93,335	41,778
Amortization related to interest rate swap agreements	361	346	226
Share based compensation expense	1,920	1,150	1,432
Hedge ineffectiveness	178	655	210
Net gain on sale of mortgage-backed securities	(64,347)	(20,576)	(13,551)
Changes in operating assets and liabilities:
Increase in accrued interest receivable	(14,088)	(25,052)	(2,413)
Increase in other assets	(191)	(1,881)	(20,399)
Increase accrued interest payable	2,996	1,419	208
Increase (decrease) in accounts payable and other liabilities	110	1,111	(721)
Net cash provided by operating activities	439,028	334,880	176,270
Investing activities
Purchases of mortgage-backed securities	(17,988,431)	(13,351,128)	(5,728,275)
Purchase of security, held to maturity			(5,000)
Principal repayments of mortgage-backed securities	5,635,988	3,559,612	2,467,288
Sale of mortgage-backed securities	4,518,105	1,766,557	563,688
Net cash used in investing activities	(7,834,338)	(8,024,959)	(2,702,299)
Financing activities
Issuance of preferred stock, net of cost of issuance	278,252
Issuance of common stock, net of cost of issuance	587,657	860,602	271,373
Cash dividends paid	(309,268)	(255,827)	(179,027)
Increase in restricted cash on swap arrangements	(44,007)	(161,592)	(14,061)
Proceeds from repurchase agreements	231,718,347	163,847,236	76,875,143
Principal repayments on repurchase agreements	(225,014,293)	(156,365,921)	(74,540,601)
Net cash provided by financing activities	7,216,688	7,924,498	2,412,827
Net (decrease) increase in cash and cash equivalents	(178,621)	234,419	(113,202)
Cash and cash equivalents, beginning of period	347,045	112,626	225,828
Cash and cash equivalents, end of period	168,424	347,045	112,626
Supplemental disclosure of cash flow information
Cash paid during the period for interest	194,068	143,243	95,715
Supplemental schedule of non-cash investing activities
Receivable for securities sold	1,587,535
Obligation to brokers for purchase of unsettled mortgage-backed securities	722,221	397,936	840,157
Preferred Stock
Supplemental schedule of non-cash financing activities
Dividends declared, not yet paid	4,628
Common Stock Amount
Supplemental schedule of non-cash financing activities
Dividends declared, not yet paid	$ 69,176	$ 69,141	$ 46,116