Valuation (Fair Value Measurement Using Significant Unobservable Inputs) (Details) (Level 3 [Member], USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	$ 708,093,000	$ 814,350,000	$ 756,934,000	$ 599,914,000
Assets, Accreted Discounts/Amortized Premiums	1,646,000	4,979,000	8,683,000	16,096,000
Assets, Realized Gain (Loss)	3,577,000	8,759,000	41,633,000	31,147,000
Assets, Change In Net Unrealized Gain/(Loss)	(3,072,000)	(1,385,000)	(12,772,000)	13,444,000
Assets, Purchases	178,574,000	153,882,000	483,619,000	521,104,000
Assets, Sales	(117,770,000)	(180,493,000)	(507,049,000)	(381,613,000)
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	771,048,000	800,092,000	771,048,000	800,092,000
Liabilities, Begining Balance	(1,275,000)	(1,709,000)	(1,333,000)	(1,336,000)
Liabilities, Accreted Discounts/Amortized Premiums	(1,000)	(10,000)	(14,000)	(31,000)
Liabilities, Realized Gain/(Loss)	(20,000)	(2,000)	(59,000)	(59,000)
Liabilities, Change In Net Unrealized Gain/(Loss)	(30,000)	252,000	(45,000)	(282,000)
Liabilities Purchases/Payments	58,000	110,000	144,000	292,000
Liabilities, Sales/Issuance	20,000	2,000	59,000	59,000
Liabilities, Transfers In and/or Out of Level 3	0	0	0	0
Liabilities, Ending Balance	(1,248,000)	(1,357,000)	(1,248,000)	(1,357,000)
Change in net unrealized gain (loss) on investments	(2,900,000)	4,300,000	2,300,000	19,600,000
Change in net unrealized gain (loss) in financial derivative assets	(700,000)	1,000,000	(1,900,000)	(8,800,000)
Change in net unrealized Gain (Loss) on financial derivative liabilities	(28,600)	2,200,000	(28,400)	(300,000)
Change in net unrealized gain (loss) in securitized debt	(2,000)	30,000	(17,000)	36,000
Securitized Debt [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liabilities, Begining Balance	(925,000)	(1,168,000)	(983,000)	(1,335,000)
Liabilities, Accreted Discounts/Amortized Premiums	(1,000)	(10,000)	(14,000)	(31,000)
Liabilities, Realized Gain/(Loss)	0	0	0	0
Liabilities, Change In Net Unrealized Gain/(Loss)	(2,000)	30,000	(17,000)	36,000
Liabilities, Payments	58,000	110,000	144,000	292,000
Liabilities, Issuances	0	0	0	0
Liabilities, Transfers In and/or Out of Level 3	0	0	0	0
Liabilities, Ending Balance	(870,000)	(1,038,000)	(870,000)	(1,038,000)
Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	693,931,000	793,216,000	740,288,000	563,883,000
Assets, Accreted Discounts/Amortized Premiums	1,646,000	4,979,000	8,683,000	16,096,000
Assets, Realized Gain (Loss)	6,437,000	11,872,000	43,768,000	31,551,000
Assets, Change In Net Unrealized Gain/(Loss)	(6,076,000)	(4,380,000)	(15,151,000)	14,941,000
Assets, Purchases	178,499,000	153,824,000	483,089,000	520,955,000
Assets, Sales	(116,177,000)	(177,034,000)	(502,417,000)	(364,949,000)
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	758,260,000	782,477,000	758,260,000	782,477,000
Investments [Member] | Agency residential mortgage-backed securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	36,240,000	27,397,000	40,504,000	6,644,000
Assets, Accreted Discounts/Amortized Premiums	(2,188,000)	(1,844,000)	(6,523,000)	(3,846,000)
Assets, Realized Gain (Loss)	286,000	0	457,000	0
Assets, Change In Net Unrealized Gain/(Loss)	492,000	875,000	515,000	1,180,000
Assets, Purchases	4,695,000	10,605,000	7,249,000	33,055,000
Assets, Sales	(953,000)	0	(3,630,000)	0
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	38,572,000	37,033,000	38,572,000	37,033,000
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	485,902,000	694,510,000	580,772,000	528,366,000
Assets, Accreted Discounts/Amortized Premiums	4,770,000	7,289,000	15,359,000	20,401,000
Assets, Realized Gain (Loss)	3,684,000	11,187,000	34,862,000	30,977,000
Assets, Change In Net Unrealized Gain/(Loss)	(5,030,000)	(5,865,000)	(14,366,000)	11,518,000
Assets, Purchases	79,431,000	106,194,000	233,654,000	363,056,000
Assets, Sales	(66,687,000)	(161,333,000)	(348,211,000)	(302,336,000)
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	502,070,000	651,982,000	502,070,000	651,982,000
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	28,047,000	23,906,000	32,994,000	19,327,000
Assets, Accreted Discounts/Amortized Premiums	145,000	161,000	497,000	258,000
Assets, Realized Gain (Loss)	1,089,000	647,000	5,479,000	(477,000)
Assets, Change In Net Unrealized Gain/(Loss)	(378,000)	139,000	(842,000)	2,612,000
Assets, Purchases	29,496,000	21,760,000	95,665,000	58,928,000
Assets, Sales	(12,082,000)	(13,342,000)	(87,476,000)	(47,377,000)
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	46,317,000	33,271,000	46,317,000	33,271,000
Investments [Member] | Commercial Mortgage Loans [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	37,415,000	7,563,000	23,887,000	9,546,000
Assets, Accreted Discounts/Amortized Premiums	832,000	31,000	1,916,000	58,000
Assets, Realized Gain (Loss)	1,029,000	0	1,901,000	482,000
Assets, Change In Net Unrealized Gain/(Loss)	(382,000)	411,000	(673,000)	497,000
Assets, Purchases	8,113,000	6,931,000	32,117,000	9,481,000
Assets, Sales	(17,155,000)	0	(29,296,000)	(5,128,000)
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	29,852,000	14,936,000	29,852,000	14,936,000
Investments [Member] | Residential Mortgage Loans [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	23,371,000		24,062,000
Assets, Accreted Discounts/Amortized Premiums	878,000		878,000
Assets, Realized Gain (Loss)	501,000		878,000
Assets, Change In Net Unrealized Gain/(Loss)	(131,000)		823,000
Assets, Purchases	(547,000)		1,148,000
Assets, Sales	(6,925,000)		(10,642,000)
Assets, Transfers In and/or Out of Level 3	0		0
Assets, Ending Balance	17,147,000		17,147,000
Investments [Member] | Other sset-backed securities and loans [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	81,315,000		38,069,000
Assets, Accreted Discounts/Amortized Premiums	(2,791,000)		(3,444,000)
Assets, Realized Gain (Loss)	(170,000)		184,000
Assets, Change In Net Unrealized Gain/(Loss)	(637,000)		(581,000)
Assets, Purchases	43,353,000		97,539,000
Assets, Sales	(11,918,000)		(22,615,000)
Assets, Transfers In and/or Out of Level 3	0		0
Assets, Ending Balance	109,152,000		109,152,000
Investments [Member] | Real Estate Owned [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	1,641,000		0
Assets, Accreted Discounts/Amortized Premiums	0		0
Assets, Realized Gain (Loss)	18,000		7,000
Assets, Change In Net Unrealized Gain/(Loss)	229,000		212,000
Assets, Purchases	6,033,000		7,792,000
Assets, Sales	(457,000)		(547,000)
Assets, Transfers In and/or Out of Level 3	0		0
Assets, Ending Balance	7,464,000		7,464,000
Investments [Member] | Other 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
Investments [Member] | Private Corporate Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	0		0
Assets, Accreted Discounts/Amortized Premiums	0		0
Assets, Realized Gain (Loss)	0		0
Assets, Change In Net Unrealized Gain/(Loss)	(239,000)		(239,000)
Assets, Purchases	7,925,000		7,925,000
Assets, Sales	0		0
Assets, Transfers In and/or Out of Level 3	0		0
Assets, Ending Balance	7,686,000		7,686,000
Financial Derivatives - Assets [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	14,162,000	21,134,000	16,646,000	36,031,000
Assets, Accreted Discounts/Amortized Premiums	0	0	0	0
Assets, Realized Gain (Loss)	(2,860,000)	(3,113,000)	(2,135,000)	(404,000)
Assets, Change In Net Unrealized Gain/(Loss)	3,004,000	2,995,000	2,379,000	(1,497,000)
Assets, Purchases	75,000	58,000	530,000	149,000
Assets, Sales	(1,593,000)	(3,459,000)	(4,632,000)	(16,664,000)
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	12,788,000	17,615,000	12,788,000	17,615,000
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	14,162,000	21,134,000	16,646,000	36,031,000
Assets, Accreted Discounts/Amortized Premiums	0	0	0	0
Assets, Realized Gain (Loss)	(2,860,000)	(3,113,000)	(2,135,000)	(404,000)
Assets, Change In Net Unrealized Gain/(Loss)	3,004,000	2,995,000	2,379,000	(1,497,000)
Assets, Purchases	75,000	58,000	530,000	149,000
Assets, Sales	(1,593,000)	(3,459,000)	(4,632,000)	(16,664,000)
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	12,788,000	17,615,000	12,788,000	17,615,000
Financial Derivatives - Liabilities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liabilities, Begining Balance	(350,000)	(541,000)	(350,000)	(1,000)
Liabilities, Accreted Discounts/Amortized Premiums	0	0	0	0
Liabilities, Realized Gain/(Loss)	(20,000)	(2,000)	(59,000)	(59,000)
Liabilities, Change In Net Unrealized Gain/(Loss)	(28,000)	222,000	(28,000)	(318,000)
Liabilities, Purchases	0	0	0	0
Liabilities, Sales	20,000	2,000	59,000	59,000
Liabilities, Transfers In and/or Out of Level 3	0	0	0	0
Liabilities, Ending Balance	(378,000)	(319,000)	(378,000)	(319,000)
Financial Derivatives - 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	(350,000)	(541,000)	(350,000)	(1,000)
Liabilities, Accreted Discounts/Amortized Premiums	0	0	0	0
Liabilities, Realized Gain/(Loss)	(20,000)	(2,000)	(59,000)	(59,000)
Liabilities, Change In Net Unrealized Gain/(Loss)	(28,000)	222,000	(28,000)	(318,000)
Liabilities, Purchases	0	0	0	0
Liabilities, Sales	20,000	2,000	59,000	59,000
Liabilities, Transfers In and/or Out of Level 3	0	0	0	0
Liabilities, Ending Balance	$ (378,000)	$ (319,000)	$ (378,000)	$ (319,000)