Financial Instruments (Carrying Values And Approximate Fair Values Of All Financial Instruments)(Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009	Dec. 31, 2008
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage backed securities	$ 17,741,873		$ 9,587,216
Unsettled purchased mortgage backed securities	49,630		49,710
Cash and cash equivalents	347,045		112,626		225,828	143,717
Restricted cash	237,014		75,422
Accrued interest receivable	63,025		37,973
Principal payments receivable	105,333		84,151
Debt Security	15,000		15,000
Interest rate hedge asset	0		23,944
Forward purchase commitments	6,327
Repurchase agreements	16,162,375		8,681,060
Payable for unsettled securities	48,999		49,774
Accrued interest payable	4,596		3,177
Interest rate hedge liability	219,167		71,681
Forward purchase commitments			8,545
Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage backed securities	17,741,873		9,587,216
Unsettled purchased mortgage backed securities	49,630		49,710
Cash and cash equivalents	347,045		112,626
Restricted cash	237,014	[1]	75,422	[1]
Accrued interest receivable	63,025		37,973
Principal payments receivable	105,333		84,151
Debt Security	15,000	[2]	15,000	[2]
Interest rate hedge asset			23,944
Short term investment	20,221	[1]	19,943	[1]
Forward purchase commitments	6,327	[1]	0	[1]
Repurchase agreements	16,162,375		8,681,060
Payable for unsettled securities	48,999		49,774
Accrued interest payable	4,596		3,177
Interest rate hedge liability	219,167		71,681
Forward purchase commitments			8,545	[2]
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage backed securities	17,741,873		9,587,216
Unsettled purchased mortgage backed securities	49,630		49,710
Cash and cash equivalents	347,045		112,626
Restricted cash	237,014	[1]	75,422	[1]
Accrued interest receivable	63,025		37,973
Principal payments receivable	105,333		84,151
Debt Security	13,811	[2]	15,000	[2]
Interest rate hedge asset			23,944
Short term investment	20,221	[1]	19,943	[1]
Forward purchase commitments	6,327	[1]	0	[1]
Repurchase agreements	16,162,375		8,681,060
Payable for unsettled securities	48,999		49,774
Accrued interest payable	4,596		3,177
Interest rate hedge liability	219,167		71,681
Forward purchase commitments			$ 8,545	[2]

[1] This line included in other assets on the balance sheet

[2] This line included in accounts payable and other liabilities on the balance sheet