Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Operating activities
Net income	$ 67,239	$ 69,304
Adjustments to reconcile net income to net cash provided by operating activities:
Net amortization of premium related to mortgage-backed securities	43,208	29,542
Amortization related to interest rate swap agreements	83	92
Share based compensation expense	629	429
Hedge ineffectiveness	116	169
Net gain on sale of mortgage-backed securities	(2,500)	(2,505)
Changes in operating assets and liabilities:
Decrease (increase) in accrued interest receivable	2,422	(240)
(Increase) decrease in other assets	37	247
Decrease in accrued interest payable	(2,552)	(1,209)
Decrease in accounts payable and other liabilities	(276)	(333)
Net cash provided by operating activities	108,406	95,496
Investing activities
Purchases of mortgage-backed securities	(1,316,188)	(2,266,198)
Principal repayments of mortgage-backed securities	1,534,530	1,066,114
Sale of mortgage-backed securities	132,630	598,164
Net cash provided by (used in) investing activities	350,972	(601,920)
Financing activities
Issuance of common stock, net of cost of issuance		562,183
Cash dividends paid	(74,656)	(69,141)
Increase (decrease) in restricted cash on swap arrangements	41,604	(12,173)
Proceeds from repurchase agreements	63,814,319	46,000,699
Principal repayments on repurchase agreements	(64,093,816)	(45,606,444)
Net cash (used in) provided by financing activities	(312,549)	875,124
Net increase in cash and cash equivalents	146,829	368,700
Cash and cash equivalents, beginning of period	168,424	347,045
Cash and cash equivalents, end of period	315,253	715,745
Supplemental disclosure of cash flow information
Cash paid during the period for interest	55,777	42,318
Supplemental schedule of non-cash investing activities
Obligation to brokers for purchase of unsettled mortgage-backed securities	1,229,848	4,226,273
Preferred Stock
Supplemental schedule of non-cash financing activities
Dividends declared, not yet paid	4,628	0
Common Stock Amount
Supplemental schedule of non-cash financing activities
Dividends declared, not yet paid	$ 69,181	$ 69,889