Consolidated Statements Of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Operating activities:
Net income	$ 45,170	$ 34,692
Noncash items:
Amortization of investment premiums	18,496	12,832
Depreciation and other amortization	48	54
Equity-based compensation costs	549	429
Change in measureable hedge ineffectiveness related to interest rate swap agreements	(318)	55
Net change in receivables, other assets, accounts payable and accrued expenses	(7,338)	(7,926)
Net cash provided by operating activities	56,607	40,136
Investing activities:
Purchases of residential mortgage investments	(1,275,570)	(2,399,175)
Purchased interest receivable related to purchases of residential mortgage investments	(2,169)	(5,419)
Principal collections on residential mortgage investments	550,133	475,039
Net cash used in investing activities	(727,606)	(1,929,555)
Financing activities:
Proceeds from repurchase arrangements and similar borrowings	32,291,152	18,400,641
Principal payments on repurchase arrangements and similar borrowings	(31,558,772)	(16,743,894)
Decrease in cash collateral receivable from interest rate swap counterparties	5,625	7,639
Increase in cash collateral payable to interest rate swap counterparties		926
Capital stock transactions	62,913	59,862
Dividends paid	(43,064)	(32,409)
Net cash provided by financing activities	757,854	1,692,765
Net change in cash and cash equivalents	86,855	(196,654)
Cash and cash equivalents at beginning of period	426,717	359,590
Cash and cash equivalents at end of period	$ 513,572	$ 162,936