Valuation (Fair Value Measurement Using Significant Unobservable Inputs) (Details) (Level 3 [Member], USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	$ 756,934,000	$ 599,914,000	$ 504,861,000
Assets, Accreted Discounts/Amortized Premiums	13,309,000	20,122,000	20,276,000
Assets, Realized Gain (Loss)	47,557,000	44,351,000	22,223,000
Assets, Change In Net Unrealized Gain/(Loss)	(20,446,000)	20,067,000	72,094,000
Assets, Purchases	782,596,000	661,133,000	454,993,000
Assets, Sales	(656,716,000)	(588,653,000)	(474,533,000)
Assets, Transfers into Level 3	0	0	0
Assets, Transfers out of Level 3	(284,748,000)	0	0
Assets, Ending Balance	638,486,000	756,934,000	599,914,000
Liabilities, Begining Balance	(1,333,000)	(1,336,000)	0
Liabilities, Accreted Discounts/Amortized Premiums	(15,000)	(38,000)	(55,000)
Liabilities, Realized Gain/(Loss)	(78,000)	(78,000)	(79,000)
Liabilities, Change In Net Unrealized Gain/(Loss)	89,000	(329,000)	(24,000)
Liabilities Purchases/Payments	246,000	370,000	265,000
Liabilities, Sales/Issuance	78,000	78,000	(1,443,000)
Liabilities, Transfers into Level 3	0	0	0
Liabilities, Transfers out of Level 3	0	0	0
Liabilities, Ending Balance	(1,013,000)	(1,333,000)	(1,336,000)
Change in net unrealized gain (loss) on investments	6,300,000	27,000,000	52,400,000
Change in net unrealized gain (loss) in financial derivative assets	(1,600,000)	(8,400,000)	(10,800,000)
Change in net unrealized Gain (Loss) on financial derivative liabilities	(100,000)	(300,000)	(1,000)
Change in net unrealized gain (loss) in securitized debt	(22,000)	20,000	(23,000)
Securitized Debt [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liabilities, Begining Balance	(983,000)	(1,335,000)	0
Liabilities, Accreted Discounts/Amortized Premiums	(15,000)	(38,000)	(55,000)
Liabilities, Realized Gain/(Loss)	0	0	0
Liabilities, Change In Net Unrealized Gain/(Loss)	(22,000)	20,000	(23,000)
Liabilities, Payments	246,000	370,000	265,000
Liabilities, Issuances	0	0	(1,522,000)
Liabilities, Transfers into Level 3	0	0	0
Liabilities, Transfers out of Level 3	0	0	0
Liabilities, Ending Balance	(774,000)	(983,000)	(1,335,000)
Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	740,288,000	563,883,000	443,363,000
Assets, Accreted Discounts/Amortized Premiums	13,309,000	20,122,000	20,276,000
Assets, Realized Gain (Loss)	52,153,000	47,177,000	23,387,000
Assets, Change In Net Unrealized Gain/(Loss)	(24,993,000)	18,997,000	68,876,000
Assets, Purchases	782,032,000	660,929,000	444,153,000
Assets, Sales	(651,042,000)	(570,820,000)	(436,172,000)
Assets, Transfers into Level 3	0	0	0
Assets, Transfers out of Level 3	(284,748,000)	0	0
Assets, Ending Balance	626,999,000	740,288,000	563,883,000
Investments [Member] | Agency residential mortgage-backed securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	40,504,000	6,644,000	5,337,000
Assets, Accreted Discounts/Amortized Premiums	(8,738,000)	(5,929,000)	(2,277,000)
Assets, Realized Gain (Loss)	1,404,000	833,000	111,000
Assets, Change In Net Unrealized Gain/(Loss)	(2,078,000)	3,323,000	(568,000)
Assets, Purchases	8,448,000	42,051,000	5,201,000
Assets, Sales	(8,155,000)	(6,418,000)	(1,160,000)
Assets, Transfers into Level 3	0	0	0
Assets, Transfers out of Level 3	0	0	0
Assets, Ending Balance	31,385,000	40,504,000	6,644,000
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	580,772,000	528,366,000	417,533,000
Assets, Accreted Discounts/Amortized Premiums	20,544,000	26,119,000	21,896,000
Assets, Realized Gain (Loss)	38,600,000	43,823,000	15,657,000
Assets, Change In Net Unrealized Gain/(Loss)	(18,406,000)	12,643,000	69,286,000
Assets, Purchases	382,956,000	524,864,000	323,554,000
Assets, Sales	(445,349,000)	(456,043,000)	(319,560,000)
Assets, Transfers into Level 3	0	0	0
Assets, Transfers out of Level 3	(284,748,000)	0	0
Assets, Ending Balance	274,369,000	580,772,000	528,366,000
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	32,994,000	19,327,000	16,093,000
Assets, Accreted Discounts/Amortized Premiums	1,328,000	402,000	570,000
Assets, Realized Gain (Loss)	7,055,000	1,030,000	7,619,000
Assets, Change In Net Unrealized Gain/(Loss)	(357,000)	3,069,000	(255,000)
Assets, Purchases	116,128,000	92,929,000	110,752,000
Assets, Sales	(103,837,000)	(83,763,000)	(115,452,000)
Assets, Transfers into Level 3	0	0	0
Assets, Transfers out of Level 3	0	0	0
Assets, Ending Balance	53,311,000	32,994,000	19,327,000
Investments [Member] | Commercial Mortgage Loans [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	23,887,000	9,546,000	4,400,000
Assets, Accreted Discounts/Amortized Premiums	2,739,000	171,000	87,000
Assets, Realized Gain (Loss)	3,156,000	844,000	0
Assets, Change In Net Unrealized Gain/(Loss)	(632,000)	652,000	413,000
Assets, Purchases	41,856,000	19,587,000	4,646,000
Assets, Sales	(42,697,000)	(6,913,000)	0
Assets, Transfers into Level 3	0	0	0
Assets, Transfers out of Level 3	0	0	0
Assets, Ending Balance	28,309,000	23,887,000	9,546,000
Investments [Member] | Residential Mortgage Loans [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	24,062,000	0
Assets, Accreted Discounts/Amortized Premiums	1,267,000	0
Assets, Realized Gain (Loss)	1,424,000	0
Assets, Change In Net Unrealized Gain/(Loss)	84,000	0
Assets, Purchases	14,863,000	24,062,000
Assets, Sales	(14,218,000)	0
Assets, Transfers into Level 3	0	0
Assets, Transfers out of Level 3	0	0
Assets, Ending Balance	27,482,000	24,062,000
Investments [Member] | Other asset-backed securities and loans [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	38,069,000
Assets, Accreted Discounts/Amortized Premiums	(3,761,000)
Assets, Realized Gain (Loss)	342,000
Assets, Change In Net Unrealized Gain/(Loss)	(2,409,000)
Assets, Purchases	148,009,000
Assets, Sales	(33,962,000)
Assets, Transfers into Level 3	0
Assets, Transfers out of Level 3	0
Assets, Ending Balance	146,288,000
Investments [Member] | Corporate Debt [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	0
Assets, Accreted Discounts/Amortized Premiums	(70,000)
Assets, Realized Gain (Loss)	0
Assets, Change In Net Unrealized Gain/(Loss)	(877,000)
Assets, Purchases	43,808,000
Assets, Sales	(153,000)
Assets, Transfers into Level 3	0
Assets, Transfers out of Level 3	0
Assets, Ending Balance	42,708,000
Investments [Member] | Real Estate Owned [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	0
Assets, Accreted Discounts/Amortized Premiums	0
Assets, Realized Gain (Loss)	172,000
Assets, Change In Net Unrealized Gain/(Loss)	(113,000)
Assets, Purchases	11,247,000
Assets, Sales	(2,671,000)
Assets, Transfers into Level 3	0
Assets, Transfers out of Level 3	0
Assets, Ending Balance	8,635,000
Investments [Member] | Other Asset-backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance		0
Assets, Accreted Discounts/Amortized Premiums		(641,000)
Assets, Realized Gain (Loss)		647,000
Assets, Change In Net Unrealized Gain/(Loss)		(690,000)
Assets, Purchases		56,436,000
Assets, Sales		(17,683,000)
Assets, Transfers into Level 3		0
Assets, Transfers out of Level 3		0
Assets, Ending Balance		38,069,000
Investments [Member] | Private Corporate Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	0
Assets, Accreted Discounts/Amortized Premiums	0
Assets, Realized Gain (Loss)	0
Assets, Change In Net Unrealized Gain/(Loss)	(205,000)
Assets, Purchases	14,717,000
Assets, Sales	0
Assets, Transfers into Level 3	0
Assets, Transfers out of Level 3	0
Assets, Ending Balance	14,512,000
Financial Derivatives - Assets [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	16,646,000	36,031,000	61,498,000
Assets, Accreted Discounts/Amortized Premiums	0	0	0
Assets, Realized Gain (Loss)	(4,596,000)	(2,826,000)	(1,164,000)
Assets, Change In Net Unrealized Gain/(Loss)	4,547,000	1,070,000	3,218,000
Assets, Purchases	564,000	204,000	10,840,000
Assets, Sales	(5,674,000)	(17,833,000)	(38,361,000)
Assets, Transfers into Level 3	0	0	0
Assets, Transfers out of Level 3	0	0	0
Assets, Ending Balance	11,487,000	16,646,000	36,031,000
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	16,646,000	36,031,000	61,498,000
Assets, Accreted Discounts/Amortized Premiums	0	0	0
Assets, Realized Gain (Loss)	(4,596,000)	(2,826,000)	(1,164,000)
Assets, Change In Net Unrealized Gain/(Loss)	4,547,000	1,070,000	3,218,000
Assets, Purchases	464,000	204,000	10,840,000
Assets, Sales	(5,674,000)	(17,833,000)	(38,361,000)
Assets, Transfers into Level 3	0	0	0
Assets, Transfers out of Level 3	0	0	0
Assets, Ending Balance	11,387,000	16,646,000	36,031,000
Financial Derivatives - Assets [Member] | Warrant [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	0
Assets, Accreted Discounts/Amortized Premiums	0
Assets, Realized Gain (Loss)	0
Assets, Change In Net Unrealized Gain/(Loss)	0
Assets, Purchases	100,000
Assets, Sales	0
Assets, Transfers into Level 3	0
Assets, Transfers out of Level 3	0
Assets, Ending Balance	100,000
Financial Derivatives - Liabilities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liabilities, Begining Balance	(350,000)	(1,000)	0
Liabilities, Accreted Discounts/Amortized Premiums	0	0	0
Liabilities, Realized Gain/(Loss)	(78,000)	(78,000)	(79,000)
Liabilities, Change In Net Unrealized Gain/(Loss)	111,000	(349,000)	(1,000)
Liabilities, Purchases	0	0	0
Liabilities, Sales	78,000	78,000	79,000
Liabilities, Transfers into Level 3	0	0	0
Liabilities, Transfers out of Level 3	0	0	0
Liabilities, Ending Balance	(239,000)	(350,000)	(1,000)
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)	(1,000)	0
Liabilities, Accreted Discounts/Amortized Premiums	0	0	0
Liabilities, Realized Gain/(Loss)	(78,000)	(78,000)	(79,000)
Liabilities, Change In Net Unrealized Gain/(Loss)	111,000	(349,000)	(1,000)
Liabilities, Purchases	0	0	0
Liabilities, Sales	78,000	78,000	79,000
Liabilities, Transfers into Level 3	0	0	0
Liabilities, Transfers out of Level 3	0	0	0
Liabilities, Ending Balance	$ (239,000)	$ (350,000)	$ (1,000)