Net Interest Income Analysis (Interest Income, Interest Expense And Weighted Average Interest Rates) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended			6 Months Ended
	Jun. 30, 2011		Jun. 30, 2010	Jun. 30, 2011		Jun. 30, 2010
Mortgage securities and similar investments	$ 63,136		$ 47,634	$ 116,277		$ 107,784
Interest Income, operating	63,194		47,769	116,448		108,011
Repurchase arrangements and similar borrowings	(13,706)		(11,146)	(26,028)		(24,514)
Unsecured borrowings	(2,187)		(2,187)	(4,374)		(4,374)
Other	(1)			(5)
Interest expense	(15,894)		(13,333)	(30,407)		(28,888)
Net interest income (expense)	47,300		34,436	86,041		79,123
Mortgage securities and similar investments, Average Rate	2.38%		2.55%	2.37%		2.83%
Other, Average Rate	0.12%		0.19%	0.16%		0.16%
Interest Income, Average Rate	2.34%		2.47%	2.32%		2.73%
Repurchase arrangements and similar borrowings, Average Rate	0.55%		0.64%	0.57%		0.69%
Unsecured Borrowings, Average rate	8.49%		8.49%	8.49%		8.49%
Other Interest Expense, Average Rate	0.10%			0.14%
Interest expenses, Average Rate	0.64%		0.76%	0.66%		0.80%
Interest income and interest expenses, Average Rate	1.70%		1.71%	1.66%		1.93%
Related Changes In Rate [Member]
Mortgage securities and similar investments	(3,353)	[1]		(19,341)	[1]
Other	(41)	[1]			[1]
Interest Income, operating	(3,394)	[1]		(19,341)	[1]
Repurchase arrangements and similar borrowings	(1,715)	[1]		(4,694)	[1]
Unsecured borrowings		[1]			[1]
Other		[1]			[1]
Interest expense	(1,715)	[1]		(4,694)	[1]
Net interest income (expense)	(1,679)	[1]		(14,647)	[1]
Related Changes In Volume [Member]
Mortgage securities and similar investments	18,855	[1]		27,834	[1]
Other	(36)	[1]		(56)	[1]
Interest Income, operating	18,819	[1]		27,778	[1]
Repurchase arrangements and similar borrowings	4,275	[1]		6,208	[1]
Unsecured borrowings		[1]			[1]
Other	1	[1]		5	[1]
Interest expense	4,276	[1]		6,213	[1]
Net interest income (expense)	14,543	[1]		21,565	[1]
Related Changes In Total [Member]
Mortgage securities and similar investments	15,502	[1]		8,493	[1]
Other	(77)	[1]		(56)	[1]
Interest Income, operating	15,425	[1]		8,437	[1]
Repurchase arrangements and similar borrowings	2,560	[1]		1,514	[1]
Unsecured borrowings		[1]			[1]
Other	1	[1]		5	[1]
Interest expense	2,561	[1]		1,519	[1]
Net interest income (expense)	$ 12,864	[1]		$ 6,918	[1]

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