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			9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010	Sep. 30, 2011		Sep. 30, 2010	Dec. 31, 2010
Net Interest Income Analysis [Line Items]
Mortgage securities and similar investments	$ 62,890		$ 40,614	$ 179,167		$ 148,398
Other	59		111	230		338
Interest Income, operating	62,949		40,725	179,397		148,736
Repurchase arrangements and similar borrowings	(15,744)		(11,096)	(41,772)		(35,610)
Unsecured borrowings	(2,186)		(2,186)	(6,560)		(6,560)
Other				(5)
Interest expense	(17,930)		(13,282)	(48,337)		(42,170)
Net interest income (expense)	45,019		27,443	131,060		106,566
Mortgage securities and similar investments, Average Rate	2.17%		2.22%	2.29%		2.63%
Other, Average Rate	0.09%		0.26%	0.13%		0.18%
Interest Income, Average Rate	2.12%		2.18%	2.25%		2.55%
Repurchase arrangements and similar borrowings, Average Rate	0.57%		0.66%	0.57%		0.68%
Unsecured Borrowings, Average rate	8.49%		8.49%	8.49%		8.49%	8.49%
Other Interest Expense, Average Rate	0.08%			0.14%
Interest expenses, Average Rate	0.65%		0.78%	0.65%		0.79%
Interest income and interest expenses, Average Rate	1.47%		1.40%	1.60%		1.76%
Related Changes In Rate [Member]
Net Interest Income Analysis [Line Items]
Mortgage securities and similar investments	(934)	[1]		(21,021)	[1]
Other	(96)	[1]		(92)	[1]
Interest Income, operating	(1,030)	[1]		(21,113)	[1]
Repurchase arrangements and similar borrowings	(1,654)	[1]		(6,356)	[1]
Unsecured borrowings		[1]			[1]
Other		[1]			[1]
Interest expense	(1,654)	[1]		(6,356)	[1]
Net interest income (expense)	624	[1]		(14,757)	[1]
Related Changes In Volume [Member]
Net Interest Income Analysis [Line Items]
Mortgage securities and similar investments	23,210	[1]		51,790	[1]
Other	44	[1]		(16)	[1]
Interest Income, operating	23,254	[1]		51,774	[1]
Repurchase arrangements and similar borrowings	6,302	[1]		12,518	[1]
Unsecured borrowings		[1]			[1]
Other		[1]		5	[1]
Interest expense	6,302	[1]		12,523	[1]
Net interest income (expense)	16,952	[1]		39,251	[1]
Related Changes In Total [Member]
Net Interest Income Analysis [Line Items]
Mortgage securities and similar investments	22,276	[1]		30,769	[1]
Other	(52)	[1]		(108)	[1]
Interest Income, operating	22,224	[1]		30,661	[1]
Repurchase arrangements and similar borrowings	4,648	[1]		6,162	[1]
Unsecured borrowings		[1]			[1]
Other		[1]		5	[1]
Interest expense	4,648	[1]		6,167	[1]
Net interest income (expense)	$ 17,576	[1]		$ 24,494	[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.