Net Interest Income Analysis (Summary Of Interest Income, Interest Expense And Weighted Average Interest Rates) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended									12 Months Ended
	Dec. 31, 2011		Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Component of Other Expense, Nonoperating [Line Items]
Mortgage securities and similar investments										$ 243,077		$ 199,300		$ 314,100
Other										301		478		495
Interest income	63,981		62,949	63,194	53,254	51,042	40,725	47,769	60,242	243,378		199,778		314,595
Repurchase arrangements and similar borrowings										(57,328)		(47,502)		(120,083)
Unsecured borrowings										(8,747)		(8,747)		(8,747)
Other										(5)		(2)
Interest expense	(17,743)		(17,930)	(15,894)	(14,513)	(14,081)	(13,282)	(13,333)	(15,555)	(66,080)		(56,251)		(128,830)
Net interest income (expense)	46,238		45,019	47,300	38,741	36,961	27,443	34,436	44,687	177,298		143,527		185,765
Mortgage securities and similar investments, Average Rate										2.24%		2.60%		4.13%
Other, Average Rate										0.12%		0.19%		0.34%
Interest income, Average Rate										2.20%		2.52%		4.06%
Repurchase arrangements and similar borrowings, Average Rate										0.56%		0.66%		1.73%
Unsecured borrowings, Average rate	8.49%	[1]				8.49%				8.49%	[1]	8.49%		8.49%
Other, Average Rate										0.14%		0.19%
Interest expense, Average Rate										0.64%		0.78%		1.83%
Interest income and interest expense, Average Rate										1.56%		1.74%		2.23%
Related Changes In Rate [Member]
Component of Other Expense, Nonoperating [Line Items]
Mortgage securities and similar investments										(30,460)	[2]	(117,267)	[2]
Other										(153)	[2]	(278)	[2]
Interest income										(30,613)	[2]	(117,545)	[2]
Repurchase arrangements and similar borrowings										(7,839)	[2]	(75,353)	[2]
Interest expense										(7,839)	[2]	(75,353)	[2]
Net interest income (expense)										(22,774)	[2]	(42,192)	[2]
Related Changes In Volume [Member]
Component of Other Expense, Nonoperating [Line Items]
Mortgage securities and similar investments										74,237	[2]	2,467	[2]
Other										(24)	[2]	261	[2]
Interest income										74,213	[2]	2,728	[2]
Repurchase arrangements and similar borrowings										17,665	[2]	2,772	[2]
Other										3	[2]	2	[2]
Interest expense										17,668	[2]	2,774	[2]
Net interest income (expense)										56,545	[2]	(46)	[2]
Related Changes In Total [Member]
Component of Other Expense, Nonoperating [Line Items]
Mortgage securities and similar investments										43,777	[2]	(114,800)	[2]
Other										(177)	[2]	(17)	[2]
Interest income										43,600	[2]	(114,817)	[2]
Repurchase arrangements and similar borrowings										9,826	[2]	(72,581)	[2]
Other										3	[2]	2	[2]
Interest expense										9,829	[2]	(72,579)	[2]
Net interest income (expense)										$ 33,771	[2]	$ (42,238)	[2]

[1] After considering cash flow hedges that coincide with the floating rate terms of these borrowings that begin in 2015 and 2016, the effective borrowing rate during the final 20 years of these borrowings will average 7.56%, subject to certain adjustments for the effects of measured hedge ineffectiveness, if any.

[2] The change in interest income and interest expense due to both rate and volume has been allocated to rate and volume changes in proportion to the relationship of the absolute dollar amounts of the change in each.