Valuation (Fair Value Measurement Using Significant Unobservable Inputs) (Details) (Level 3 [Member], USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	$ 627,575,000	$ 479,663,000	$ 599,914,000	$ 504,861,000
Assets, Accreted Discounts/Amortized Premiums	5,331,000	4,522,000	11,117,000	8,417,000
Assets, Realized Gain (Loss)	4,984,000	79,000	22,388,000	1,881,000
Assets, Change In Net Unrealized Gain/(Loss)	(7,125,000)	7,711,000	14,829,000	34,361,000
Assets, Purchases	259,987,000	81,451,000	367,223,000	182,432,000
Assets, Sales	(76,402,000)	(111,886,000)	(201,121,000)	(270,412,000)
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	814,350,000	461,540,000	814,350,000	461,540,000
Liabilities, Begining Balance	(1,207,000)		(1,336,000)
Liabilities, Accreted Discounts/Amortized Premiums	(11,000)		(20,000)
Liabilities, Realized Gain/(Loss)	(29,000)		(57,000)
Liabilities, Change In Net Unrealized Gain/(Loss)	(603,000)		(535,000)
Liabilities Purchases/Payments	111,000		182,000
Liabilities, Sales/Issuance	30,000		57,000
Liabilities, Transfers In and/or Out of Level 3	0		0
Liabilities, Ending Balance	(1,709,000)		(1,709,000)
Change in net unrealized gain (loss) on investments	(5,000,000)	1,600,000	21,500,000	11,900,000
Change in net unrealized gain (loss) in financial derivative assets	1,700,000	1,300,000	(9,100,000)	(5,500,000)
Change in net unrealized Gain (Loss) on financial derivative liabilities	(1,100,000)		(1,100,000)
Change in net unrealized gain (loss) in securitized debt	(63,000)	10,000	5,000	21,000
Securitized Debt [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liabilities, Begining Balance	(1,205,000)	(1,485,000)	(1,335,000)	0
Liabilities, Accreted Discounts/Amortized Premiums	(11,000)	(15,000)	(20,000)	(29,000)
Liabilities, Realized Gain/(Loss)	0	0	0	0
Liabilities, Change In Net Unrealized Gain/(Loss)	(63,000)	10,000	5,000	21,000
Liabilities, Payments	111,000	75,000	182,000	115,000
Liabilities, Issuances	0	0	0	(1,522,000)
Liabilities, Transfers In and/or Out of Level 3	0	0	0	0
Liabilities, Ending Balance	(1,168,000)	(1,415,000)	(1,168,000)	(1,415,000)
Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	601,177,000	430,917,000	563,883,000	443,363,000
Assets, Accreted Discounts/Amortized Premiums	5,331,000	4,522,000	11,117,000	8,417,000
Assets, Realized Gain (Loss)	5,526,000	813,000	19,679,000	7,358,000
Assets, Change In Net Unrealized Gain/(Loss)	(8,232,000)	6,403,000	19,321,000	26,254,000
Assets, Purchases	259,945,000	81,348,000	367,132,000	182,206,000
Assets, Sales	(70,531,000)	(101,222,000)	(187,916,000)	(244,817,000)
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	793,216,000	422,781,000	793,216,000	422,781,000
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	16,228,000	6,016,000	6,644,000	5,337,000
Assets, Accreted Discounts/Amortized Premiums	(1,132,000)	(553,000)	(2,003,000)	(1,177,000)
Assets, Realized Gain (Loss)	0	0	0	0
Assets, Change In Net Unrealized Gain/(Loss)	68,000	(796,000)	306,000	(365,000)
Assets, Purchases	12,233,000	0	22,450,000	872,000
Assets, Sales	0	0	0	0
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	27,397,000	4,667,000	27,397,000	4,667,000
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	557,820,000	408,230,000	528,366,000	417,533,000
Assets, Accreted Discounts/Amortized Premiums	6,471,000	4,914,000	13,112,000	(9,288,000)
Assets, Realized Gain (Loss)	4,189,000	848,000	19,790,000	7,049,000
Assets, Change In Net Unrealized Gain/(Loss)	(6,636,000)	7,475,000	17,383,000	25,136,000
Assets, Purchases	184,029,000	59,941,000	256,862,000	158,618,000
Assets, Sales	(51,363,000)	(96,349,000)	(141,003,000)	(232,565,000)
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	694,510,000	385,059,000	694,510,000	385,059,000
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	5,778,000	12,171,000	19,327,000	16,093,000
Assets, Accreted Discounts/Amortized Premiums	41,000	139,000	98,000	256,000
Assets, Realized Gain (Loss)	324,000	(35,000)	(1,124,000)	309,000
Assets, Change In Net Unrealized Gain/(Loss)	(771,000)	(404,000)	2,473,000	1,283,000
Assets, Purchases	24,826,000	21,407,000	37,168,000	22,716,000
Assets, Sales	(6,292,000)	(4,873,000)	(34,036,000)	(12,252,000)
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	23,906,000	28,405,000	23,906,000	28,405,000
Investments [Member] | Commercial Mortgage Loans [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	9,713,000	4,500,000	9,546,000	4,400,000
Assets, Accreted Discounts/Amortized Premiums	19,000	22,000	27,000	50,000
Assets, Realized Gain (Loss)	482,000	0	482,000	0
Assets, Change In Net Unrealized Gain/(Loss)	(73,000)	128,000	86,000	200,000
Assets, Purchases	2,550,000	0	2,551,000	0
Assets, Sales	(5,128,000)	0	(5,129,000)	0
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	7,563,000	4,650,000	7,563,000	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	11,638,000		0
Assets, Accreted Discounts/Amortized Premiums	(68,000)		(117,000)
Assets, Realized Gain (Loss)	531,000		531,000
Assets, Change In Net Unrealized Gain/(Loss)	(820,000)		(927,000)
Assets, Purchases	36,307,000		48,101,000
Assets, Sales	(7,748,000)		(7,748,000)
Assets, Transfers In and/or Out of Level 3	0		0
Assets, Ending Balance	39,840,000		39,840,000
Financial Derivatives - Assets [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	26,398,000	48,746,000	36,031,000	61,498,000
Assets, Accreted Discounts/Amortized Premiums	0	0	0	0
Assets, Realized Gain (Loss)	(542,000)	(734,000)	2,709,000	(5,477,000)
Assets, Change In Net Unrealized Gain/(Loss)	1,107,000	1,308,000	(4,492,000)	8,107,000
Assets, Purchases	42,000	103,000	91,000	226,000
Assets, Sales	(5,871,000)	(10,664,000)	(13,205,000)	(25,595,000)
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	21,134,000	38,759,000	21,134,000	38,759,000
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	26,398,000	48,746,000	36,031,000	61,498,000
Assets, Accreted Discounts/Amortized Premiums	0	0	0	0
Assets, Realized Gain (Loss)	(542,000)	(734,000)	2,709,000	(5,477,000)
Assets, Change In Net Unrealized Gain/(Loss)	1,107,000	1,308,000	(4,492,000)	8,107,000
Assets, Purchases	42,000	103,000	91,000	226,000
Assets, Sales	(5,871,000)	(10,664,000)	(13,205,000)	(25,595,000)
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	21,134,000	38,759,000	21,134,000	38,759,000
Derivative Financial Instruments, Liabilities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liabilities, Begining Balance	(2,000)		(1,000)
Liabilities, Accreted Discounts/Amortized Premiums	0		0
Liabilities, Realized Gain/(Loss)	(29,000)		(57,000)
Liabilities, Change In Net Unrealized Gain/(Loss)	(540,000)		(540,000)
Liabilities, Purchases	0		0
Liabilities, Sales	30,000		57,000
Liabilities, Transfers In and/or Out of Level 3	0		0
Liabilities, Ending Balance	(541,000)		(541,000)
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	(2,000)		(1,000)
Liabilities, Accreted Discounts/Amortized Premiums	0		0
Liabilities, Realized Gain/(Loss)	(29,000)		(57,000)
Liabilities, Change In Net Unrealized Gain/(Loss)	(540,000)		(540,000)
Liabilities, Purchases	0		0
Liabilities, Sales	30,000		57,000
Liabilities, Transfers In and/or Out of Level 3	0		0
Liabilities, Ending Balance	$ (541,000)		$ (541,000)