Consolidated 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	$ 160,204	$ 126,896	$ 129,263
Noncash items:
Amortization of investment premiums	68,077	57,634	29,426
Depreciation and other amortization	206	220	266
Equity-based compensation costs	1,821	1,525	1,098
Amounts related to interest rate swap agreements	306	(722)	1,909
Impairment charges related to real estate held for sale	470	106	40,423
Gain on asset sales	(114)	(277)	(23)
Net change in mortgage securities principal remittance receivable	(17,478)	5,508	(14,019)
Net change in remaining receivables, other assets, accounts payable and accrued expenses	6,743	6,989	(5,229)
Net cash provided by operating activities	220,235	197,879	183,114
Investing activities:
Purchases of mortgage securities and similar investments	(5,899,041)	(3,414,143)	(2,014,827)
Purchased interest receivable related to purchases of mortgage securities and similar investments	(12,099)	(6,616)	(4,546)
Proceeds from asset sales	10,786	4,131	541
Principal collections on mortgage securities and similar investments	2,127,812	2,932,978	1,513,967
Net cash used in investing activities	(3,772,542)	(483,650)	(504,865)
Financing activities:
Proceeds from repurchase arrangements and similar borrowings	88,814,514	69,438,636	65,730,943
Principal payments on repurchase arrangements and similar borrowings	(85,254,811)	(69,081,148)	(65,047,179)
Decrease (increase) in cash collateral receivable from interest rate swap counterparties	(13,216)	(4,804)	23,191
Increase (decrease) in cash collateral payable to interest rate swap counterparties	(9,024)	9,024
Capital stock transactions	236,811	10,233	81,696
Dividends paid	(154,840)	(136,203)	(154,116)
Net cash provided by financing activities	3,619,434	235,738	634,535
Net change in cash and cash equivalents	67,127	(50,033)	312,784
Cash and cash equivalents at beginning of year	359,590	409,623	96,839
Cash and cash equivalents at end of year	$ 426,717	$ 359,590	$ 409,623