Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Operating activities
Net income	$ 242,498	$ 213,749
Adjustments to reconcile net income to net cash provided by operating activities:
Net amortization of premium related to mortgage-backed securities	112,736	60,719
Amortization related to interest rate swap agreements	275	253
Share based compensation expense	1,301	735
Hedge ineffectiveness	321	516
Net gain on sale of mortgage-backed securities	(25,244)	(17,735)
Changes in operating assets and liabilities:
Increase in accrued interest receivable	(17,031)	(25,900)
Increase in other assets	(1,907)	(1,199)
(Decrease) increase in accrued interest payable	133	(849)
(Decrease) increase in accounts payable and other liabilities	(194)	165
Net cash provided by operating activities	312,888	230,454
Investing activities
Purchases of mortgage-backed securities	(15,160,173)	(11,327,766)
Principal repayments of mortgage-backed securities	3,978,718	2,360,742
Sale of mortgage-backed securities	2,678,086	1,037,064
Net cash used in investing activities	(8,503,369)	(7,929,960)
Financing activities
Issuance of preferred stock, net of cost of issuance	278,263
Issuance of common stock, net of cost of issuance	587,677	853,441
Cash dividends paid	(227,297)	(179,280)
Increase in restricted cash on swap arrangements	(65,166)	(204,114)
Proceeds from repurchase agreements	170,030,964	118,620,023
Principal repayments on repurchase agreements	(162,610,159)	(111,414,852)
Net cash provided by financing activities	7,994,282	7,675,218
Net decrease in cash and cash equivalents	(196,199)	(24,288)
Cash and cash equivalents, beginning of period	347,045	112,626
Cash and cash equivalents, end of period	150,846	88,338
Supplemental disclosure of cash flow information
Cash paid during the period for interest	140,012	103,211
Supplemental schedule of non-cash investing activities
Obligation to brokers for purchase of unsettled mortgage-backed securities	1,527,972	1,395,612
Preferred Stock
Supplemental schedule of non-cash financing activities
Dividends declared, not yet paid	2,070
Common Stock Amount
Supplemental schedule of non-cash financing activities
Dividends declared, not yet paid	$ 79,048	$ 76,547