Valuation (Fair Value Measurement Using Significant Unobservable Inputs) (Details) (Level 3 [Member], USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	$ 814,350,000	$ 461,540,000	$ 599,914,000	$ 504,861,000
Assets, Accreted Discounts/Amortized Premiums	4,979,000	5,940,000	16,096,000	14,358,000
Assets, Realized Gain (Loss)	8,759,000	12,419,000	31,147,000	14,299,000
Assets, Change In Net Unrealized Gain/(Loss)	(1,385,000)	19,562,000	13,444,000	53,924,000
Assets, Purchases	153,882,000	150,769,000	521,104,000	333,200,000
Assets, Sales	(180,493,000)	(103,172,000)	(381,613,000)	(373,584,000)
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	800,092,000	547,058,000	800,092,000	547,058,000
Liabilities, Begining Balance	(1,709,000)		(1,336,000)
Liabilities, Accreted Discounts/Amortized Premiums	(10,000)		(31,000)
Liabilities, Realized Gain/(Loss)	(2,000)		(59,000)
Liabilities, Change In Net Unrealized Gain/(Loss)	252,000		(282,000)
Liabilities Purchases/Payments	110,000		292,000
Liabilities, Sales/Issuance	2,000		59,000
Liabilities, Transfers In and/or Out of Level 3	0		0
Liabilities, Ending Balance	(1,357,000)		(1,357,000)
Change in net unrealized gain (loss) on investments	4,300,000	23,300,000	19,600,000	33,700,000
Change in net unrealized gain (loss) in financial derivative assets	1,000,000	(3,800,000)	(8,800,000)	(8,100,000)
Change in net unrealized Gain (Loss) on financial derivative liabilities	2,200,000		(300,000)
Change in net unrealized gain (loss) in securitized debt	30,000	(51,000)	36,000	(30,000)
Securitized Debt [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liabilities, Begining Balance	(1,168,000)	(1,415,000)	(1,335,000)	0
Liabilities, Accreted Discounts/Amortized Premiums	(10,000)	(15,000)	(31,000)	(44,000)
Liabilities, Realized Gain/(Loss)	0	0	0	0
Liabilities, Change In Net Unrealized Gain/(Loss)	30,000	(51,000)	36,000	(30,000)
Liabilities, Payments	110,000	42,000	292,000	157,000
Liabilities, Issuances	0	0	0	(1,522,000)
Liabilities, Transfers In and/or Out of Level 3	0	0	0	0
Liabilities, Ending Balance	(1,038,000)	(1,439,000)	(1,038,000)	(1,439,000)
Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	793,216,000	422,781,000	563,883,000	443,363,000
Assets, Accreted Discounts/Amortized Premiums	4,979,000	5,940,000	16,096,000	14,358,000
Assets, Realized Gain (Loss)	11,872,000	8,456,000	31,551,000	15,813,000
Assets, Change In Net Unrealized Gain/(Loss)	(4,380,000)	23,385,000	14,941,000	49,639,000
Assets, Purchases	153,824,000	150,664,000	520,955,000	332,870,000
Assets, Sales	(177,034,000)	(95,628,000)	(364,949,000)	(340,445,000)
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	782,477,000	515,598,000	782,477,000	515,598,000
Investments [Member] | U.S. Treasury And Agency Residential Mortgage-Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	27,397,000	4,667,000	6,644,000	5,337,000
Assets, Accreted Discounts/Amortized Premiums	(1,844,000)	(534,000)	(3,846,000)	(1,177,000)
Assets, Realized Gain (Loss)	0	111,000	0	111,000
Assets, Change In Net Unrealized Gain/(Loss)	875,000	(314,000)	1,180,000	(679,000)
Assets, Purchases	10,605,000	1,057,000	33,055,000	1,929,000
Assets, Sales	0	(1,160,000)	0	(1,160,000)
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	37,033,000	3,827,000	37,033,000	3,827,000
Investments [Member] | Private Label Residential Mortgage-Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	694,510,000	385,059,000	528,366,000	417,533,000
Assets, Accreted Discounts/Amortized Premiums	7,289,000	6,182,000	20,401,000	15,471,000
Assets, Realized Gain (Loss)	11,187,000	5,440,000	30,977,000	12,489,000
Assets, Change In Net Unrealized Gain/(Loss)	(5,865,000)	24,013,000	11,518,000	49,149,000
Assets, Purchases	106,194,000	92,412,000	363,056,000	251,030,000
Assets, Sales	(161,333,000)	(55,181,000)	(302,336,000)	(287,747,000)
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	651,982,000	457,925,000	651,982,000	457,925,000
Investments [Member] | Private Label Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	23,906,000	28,405,000	19,327,000	16,093,000
Assets, Accreted Discounts/Amortized Premiums	161,000	271,000	258,000	527,000
Assets, Realized Gain (Loss)	647,000	2,905,000	(477,000)	3,213,000
Assets, Change In Net Unrealized Gain/(Loss)	139,000	(293,000)	2,612,000	990,000
Assets, Purchases	21,760,000	57,195,000	58,928,000	79,911,000
Assets, Sales	(13,342,000)	(39,287,000)	(47,377,000)	(51,538,000)
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	33,271,000	49,196,000	33,271,000	49,196,000
Investments [Member] | Commercial Mortgage Loans [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	7,563,000	4,650,000	9,546,000	4,400,000
Assets, Accreted Discounts/Amortized Premiums	31,000	21,000	58,000	71,000
Assets, Realized Gain (Loss)	0	0	482,000	0
Assets, Change In Net Unrealized Gain/(Loss)	411,000	(21,000)	497,000	179,000
Assets, Purchases	6,931,000	0	9,481,000	0
Assets, Sales	0	0	(5,128,000)	0
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	14,936,000	4,650,000	14,936,000	4,650,000
Investments [Member] | Asset-backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	39,840,000		0
Assets, Accreted Discounts/Amortized Premiums	(658,000)		(775,000)
Assets, Realized Gain (Loss)	38,000		569,000
Assets, Change In Net Unrealized Gain/(Loss)	60,000		(866,000)
Assets, Purchases	8,334,000		56,435,000
Assets, Sales	(2,359,000)		(10,108,000)
Assets, Transfers In and/or Out of Level 3	0		0
Assets, Ending Balance	45,255,000		45,255,000
Financial Derivatives - Assets [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	21,134,000	38,759,000	36,031,000	61,498,000
Assets, Accreted Discounts/Amortized Premiums	0	0	0	0
Assets, Realized Gain (Loss)	(3,113,000)	3,963,000	(404,000)	(1,514,000)
Assets, Change In Net Unrealized Gain/(Loss)	2,995,000	(3,823,000)	(1,497,000)	4,285,000
Assets, Purchases	58,000	105,000	149,000	330,000
Assets, Sales	(3,459,000)	(7,544,000)	(16,664,000)	(33,139,000)
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	17,615,000	31,460,000	17,615,000	31,460,000
Financial Derivatives - Assets [Member] | Credit Default Swaps On Asset Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	21,134,000	38,759,000	36,031,000	61,498,000
Assets, Accreted Discounts/Amortized Premiums	0	0	0	0
Assets, Realized Gain (Loss)	(3,113,000)	3,963,000	(404,000)	(1,514,000)
Assets, Change In Net Unrealized Gain/(Loss)	2,995,000	(3,823,000)	(1,497,000)	4,285,000
Assets, Purchases	58,000	105,000	149,000	330,000
Assets, Sales	(3,459,000)	(7,544,000)	(16,664,000)	(33,139,000)
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	17,615,000	31,460,000	17,615,000	31,460,000
Derivative Financial Instruments, Liabilities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liabilities, Begining Balance	(541,000)		(1,000)
Liabilities, Accreted Discounts/Amortized Premiums	0		0
Liabilities, Realized Gain/(Loss)	(2,000)		(59,000)
Liabilities, Change In Net Unrealized Gain/(Loss)	222,000		(318,000)
Liabilities, Purchases	0		0
Liabilities, Sales	2,000		59,000
Liabilities, Transfers In and/or Out of Level 3	0		0
Liabilities, Ending Balance	(319,000)		(319,000)
Derivative Financial Instruments, Liabilities [Member] | Credit Default Swaps On Asset Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liabilities, Begining Balance	(541,000)		(1,000)
Liabilities, Accreted Discounts/Amortized Premiums	0		0
Liabilities, Realized Gain/(Loss)	(2,000)		(59,000)
Liabilities, Change In Net Unrealized Gain/(Loss)	222,000		(318,000)
Liabilities, Purchases	0		0
Liabilities, Sales	2,000		59,000
Liabilities, Transfers In and/or Out of Level 3	0		0
Liabilities, Ending Balance	$ (319,000)		$ (319,000)