Schedule of Derivative Instruments in Statement of Financial Position, Fair Value (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Derivative Financial Instruments, Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	$ 429,419	$ 549,859
Derivative, Notional Amount	45,175,522	30,383,908
Derivative Financial Instruments, Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(8,395)	(22,081)
Derivative, Notional Amount	2,894,624	10,876,702
Inverse Interest-Only Securities [Member] | Derivative Financial Instruments, Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	212,984	221,364
Derivative, Notional Amount	1,412,374	1,525,845
Inverse Interest-Only Securities [Member] | Derivative Financial Instruments, Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	0	0
Derivative, Notional Amount	0	0
Interest Rate Swap [Member] | Derivative Financial Instruments, Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	40,310	25,325
Derivative, Notional Amount	21,663,148	19,619,000
Interest Rate Swap [Member] | Derivative Financial Instruments, Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	0	0
Derivative, Notional Amount	0	0
Credit Default Swap [Member] | Derivative Financial Instruments, Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	0	0
Derivative, Notional Amount	0	0
Credit Default Swap [Member] | Derivative Financial Instruments, Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(2,395)	(18,049)
Derivative, Notional Amount	125,000	427,073
Interest Rate Swaption [Member] | Derivative Financial Instruments, Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	164,296	269,745
Derivative, Notional Amount	9,500,000	5,130,000
Interest Rate Swaption [Member] | Derivative Financial Instruments, Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	0	0
Derivative, Notional Amount	0	0
Forward Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	(1,022,000)	603,000
Forward Contracts [Member] | Derivative Financial Instruments, Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	4,331	33,425
Derivative, Notional Amount	1,100,000	4,097,000
Forward Contracts [Member] | Derivative Financial Instruments, Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(5,487)	(125)
Derivative, Notional Amount	2,372,000	400,000
Options Held [Member] | Derivative Financial Instruments, Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	3,627
Derivative, Notional Amount	1,500,000
Options Held [Member] | Derivative Financial Instruments, Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	0
Derivative, Notional Amount	0
Swap [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	3,871	(3,773)
Derivative, Notional Amount	10,000,000	10,000,000
Swap [Member] | Derivative Financial Instruments, Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	3,871	0
Derivative, Notional Amount	10,000,000	0
Swap [Member] | Derivative Financial Instruments, Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	0	(3,773)
Derivative, Notional Amount	0	10,000,000
Total Return Swap [Member] | Derivative Financial Instruments, Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	0	0
Derivative, Notional Amount	0	0
Total Return Swap [Member] | Derivative Financial Instruments, Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(126)	(134)
Derivative, Notional Amount	243,987	49,629
Forward Purchase Commitment [Member] | Derivative Financial Instruments, Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	0	0
Derivative, Notional Amount	0	12,063
Forward Purchase Commitment [Member] | Derivative Financial Instruments, Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(387)	0
Derivative, Notional Amount	$ 153,637	$ 0