Valuation (Fair Value Measurement Using Significant Unobservable Inputs) (Details) (Level 3 [Member], USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	$ 599,914,000	$ 504,861,000	$ 443,539,000
Assets, Accreted Discounts/Amortized Premiums	20,122,000	20,276,000	15,455,000
Assets, Realized Gain (Loss)	44,351,000	22,223,000	28,433,000
Assets, Change In Net Unrealized Gain/(Loss)	20,067,000	72,094,000	(54,734,000)
Assets, Purchases	661,133,000	454,993,000	390,471,000
Assets, Sales	(588,653,000)	(474,533,000)	(318,303,000)
Assets, Transfers In and/or Out of Level 3	0	0	0
Assets, Ending Balance	756,934,000	599,914,000	504,861,000
Liabilities, Begining Balance	(1,336,000)	0
Liabilities, Accreted Discounts/Amortized Premiums	(38,000)	(55,000)
Liabilities, Realized Gain/(Loss)	(78,000)	(79,000)
Liabilities, Change In Net Unrealized Gain/(Loss)	(329,000)	(24,000)
Liabilities Purchases/Payments	370,000	265,000
Liabilities, Sales/Issuance	78,000	(1,443,000)
Liabilities, Transfers In and/or Out of Level 3	0	0
Liabilities, Ending Balance	(1,333,000)	(1,336,000)	0
Change in net unrealized gain (loss) on investments	27,000,000	52,400,000	(39,600,000)
Change in net unrealized gain (loss) in financial derivative assets	(8,400,000)	(10,800,000)	(14,200,000)
Change in net unrealized Gain (Loss) on financial derivative liabilities	(300,000)	(1,000)
Change in net unrealized gain (loss) in securitized debt	20,000	(23,000)
Securitized Debt [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liabilities, Begining Balance	(1,335,000)	0
Liabilities, Accreted Discounts/Amortized Premiums	(38,000)	(55,000)
Liabilities, Realized Gain/(Loss)	0	0
Liabilities, Change In Net Unrealized Gain/(Loss)	20,000	(23,000)
Liabilities, Payments	370,000	265,000
Liabilities, Issuances	0	(1,522,000)
Liabilities, Transfers In and/or Out of Level 3	0	0
Liabilities, Ending Balance	(983,000)	(1,335,000)
Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	563,883,000	443,363,000	340,689,000
Assets, Accreted Discounts/Amortized Premiums	20,122,000	20,276,000	15,455,000
Assets, Realized Gain (Loss)	47,177,000	23,387,000	18,464,000
Assets, Change In Net Unrealized Gain/(Loss)	18,997,000	68,876,000	(49,013,000)
Assets, Purchases	660,929,000	444,153,000	384,529,000
Assets, Sales	(570,820,000)	(436,172,000)	(266,761,000)
Assets, Transfers In and/or Out of Level 3	0	0	0
Assets, Ending Balance	740,288,000	563,883,000	443,363,000
Investments [Member] | Agency residential mortgage-backed securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	6,644,000	5,337,000	0
Assets, Accreted Discounts/Amortized Premiums	(5,929,000)	(2,277,000)	(1,457,000)
Assets, Realized Gain (Loss)	833,000	111,000	97,000
Assets, Change In Net Unrealized Gain/(Loss)	3,323,000	(568,000)	(2,078,000)
Assets, Purchases	42,051,000	5,201,000	9,930,000
Assets, Sales	(6,418,000)	(1,160,000)	(1,155,000)
Assets, Transfers In and/or Out of Level 3	0	0	0
Assets, Ending Balance	40,504,000	6,644,000	5,337,000
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	528,366,000	417,533,000	338,839,000
Assets, Accreted Discounts/Amortized Premiums	26,119,000	21,896,000	16,259,000
Assets, Realized Gain (Loss)	43,823,000	15,657,000	17,246,000
Assets, Change In Net Unrealized Gain/(Loss)	12,643,000	69,286,000	(43,559,000)
Assets, Purchases	425,864,000	323,554,000	342,360,000
Assets, Sales	(456,043,000)	(319,560,000)	(253,612,000)
Assets, Transfers In and/or Out of Level 3	0	0	0
Assets, Ending Balance	580,772,000	528,366,000	417,533,000
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	19,327,000	16,093,000	1,850,000
Assets, Accreted Discounts/Amortized Premiums	402,000	570,000	539,000
Assets, Realized Gain (Loss)	1,030,000	7,619,000	1,121,000
Assets, Change In Net Unrealized Gain/(Loss)	3,069,000	(255,000)	(2,987,000)
Assets, Purchases	92,929,000	110,752,000	27,564,000
Assets, Sales	(83,763,000)	(115,452,000)	(11,994,000)
Assets, Transfers In and/or Out of Level 3	0	0	0
Assets, Ending Balance	32,994,000	19,327,000	16,093,000
Investments [Member] | Commercial Mortgage Loans [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	9,546,000	4,400,000	0
Assets, Accreted Discounts/Amortized Premiums	171,000	87,000	114,000
Assets, Realized Gain (Loss)	844,000	0	0
Assets, Change In Net Unrealized Gain/(Loss)	652,000	413,000	(389,000)
Assets, Purchases	19,587,000	4,646,000	4,675,000
Assets, Sales	(6,913,000)	0	0
Assets, Transfers In and/or Out of Level 3	0	0	0
Assets, Ending Balance	23,887,000	9,546,000	4,400,000
Investments [Member] | Residential Mortgage Loans [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	24,062,000
Assets, Sales	0
Assets, Transfers In and/or Out of Level 3	0
Assets, Ending Balance	24,062,000
Investments [Member] | 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 In and/or Out of Level 3	0
Assets, Ending Balance	38,069,000
Financial Derivatives - Assets [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	36,031,000	61,498,000	102,850,000
Assets, Accreted Discounts/Amortized Premiums	0	0	0
Assets, Realized Gain (Loss)	(2,826,000)	(1,164,000)	9,969,000
Assets, Change In Net Unrealized Gain/(Loss)	1,070,000	3,218,000	(5,721,000)
Assets, Purchases	204,000	10,840,000	5,942,000
Assets, Sales	(17,833,000)	(38,361,000)	(51,542,000)
Assets, Transfers In and/or Out of Level 3	0	0	0
Assets, Ending Balance	16,646,000	36,031,000	61,498,000
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	36,031,000	61,498,000	102,850,000
Assets, Accreted Discounts/Amortized Premiums	0	0	0
Assets, Realized Gain (Loss)	(2,826,000)	(1,164,000)	9,969,000
Assets, Change In Net Unrealized Gain/(Loss)	1,070,000	3,218,000	(5,721,000)
Assets, Purchases	204,000	10,840,000	5,942,000
Assets, Sales	(17,833,000)	(38,361,000)	(51,542,000)
Assets, Transfers In and/or Out of Level 3	0	0	0
Assets, Ending Balance	16,646,000	36,031,000	61,498,000
Derivative Financial Instruments, Liabilities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liabilities, Begining Balance	(1,000)	0
Liabilities, Accreted Discounts/Amortized Premiums	0	0
Liabilities, Realized Gain/(Loss)	(78,000)	(79,000)
Liabilities, Change In Net Unrealized Gain/(Loss)	(349,000)	(1,000)
Liabilities, Purchases	0	0
Liabilities, Sales	78,000	79,000
Liabilities, Transfers In and/or Out of Level 3	0	0
Liabilities, Ending Balance	(350,000)	(1,000)
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	(1,000)	0
Liabilities, Accreted Discounts/Amortized Premiums	0	0
Liabilities, Realized Gain/(Loss)	(78,000)	(79,000)
Liabilities, Change In Net Unrealized Gain/(Loss)	(349,000)	(1,000)
Liabilities, Purchases	0	0
Liabilities, Sales	78,000	79,000
Liabilities, Transfers In and/or Out of Level 3	0	0
Liabilities, Ending Balance	$ (350,000)	$ (1,000)