Valuation (Fair Value Measurement Using Significant Unobservable Inputs) (Details) (Level 3 [Member], USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets, Fair Value Disclosure	$ 694,403,000		$ 756,934,000
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	756,934,000	599,914,000
Assets, Accreted Discounts/Amortized Premiums	4,023,000	5,786,000
Assets, Realized Gain (Loss)	25,480,000	17,404,000
Assets, Change In Net Unrealized Gain/(Loss)	(11,972,000)	21,955,000
Assets, Purchases	163,958,000	107,235,000
Assets, Sales	(244,020,000)	(124,719,000)
Assets, Transfers In and/or Out of Level 3	0	0
Assets, Ending Balance	694,403,000	627,575,000
Liabilities, Begining Balance	(1,333,000)	(1,336,000)
Liabilities, Accreted Discounts/Amortized Premiums	(7,000)	(9,000)
Liabilities, Realized Gain/(Loss)	(19,000)	(28,000)
Liabilities, Change In Net Unrealized Gain/(Loss)	(17,000)	68,000
Liabilities Purchases/Payments	24,000	70,000
Liabilities, Sales/Issuance	19,000	28,000
Liabilities, Transfers In and/or Out of Level 3	0	0
Liabilities, Ending Balance	(1,333,000)	(1,207,000)
Change in net unrealized gain (loss) on investments	2,300,000	28,300,000
Change in net unrealized gain (loss) in financial derivative assets	(1,200,000)	(7,200,000)
Change in net unrealized Gain (Loss) on financial derivative liabilities		(1,000)
Change in net unrealized gain (loss) in securitized debt	(17,000)	70,000
Securitized Debt [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liabilities, Begining Balance	(983,000)	(1,335,000)
Liabilities, Accreted Discounts/Amortized Premiums	(7,000)	(9,000)
Liabilities, Realized Gain/(Loss)	0	0
Liabilities, Change In Net Unrealized Gain/(Loss)	(17,000)	69,000
Liabilities, Payments	24,000	70,000
Liabilities, Issuances	0	0
Liabilities, Transfers In and/or Out of Level 3	0	0
Liabilities, Ending Balance	(983,000)	(1,205,000)
Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	740,288,000	563,883,000
Assets, Accreted Discounts/Amortized Premiums	4,023,000	5,786,000
Assets, Realized Gain (Loss)	24,290,000	14,153,000
Assets, Change In Net Unrealized Gain/(Loss)	(10,762,000)	27,554,000
Assets, Purchases	163,919,000	107,186,000
Assets, Sales	(242,529,000)	(117,385,000)
Assets, Transfers In and/or Out of Level 3	0	0
Assets, Ending Balance	679,229,000	601,177,000
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
Assets, Accreted Discounts/Amortized Premiums	(2,289,000)	(870,000)
Assets, Realized Gain (Loss)	35,000	0
Assets, Change In Net Unrealized Gain/(Loss)	751,000	238,000
Assets, Purchases	2,554,000	10,216,000
Assets, Sales	(180,000)	0
Assets, Transfers In and/or Out of Level 3	0	0
Assets, Ending Balance	41,375,000	16,228,000
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
Assets, Accreted Discounts/Amortized Premiums	5,385,000	6,640,000
Assets, Realized Gain (Loss)	23,460,000	15,602,000
Assets, Change In Net Unrealized Gain/(Loss)	(12,852,000)	24,020,000
Assets, Purchases	89,585,000	72,832,000
Assets, Sales	(196,267,000)	(89,640,000)
Assets, Transfers In and/or Out of Level 3	0	0
Assets, Ending Balance	490,083,000	557,820,000
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
Assets, Accreted Discounts/Amortized Premiums	205,000	57,000
Assets, Realized Gain (Loss)	872,000	(1,449,000)
Assets, Change In Net Unrealized Gain/(Loss)	1,515,000	3,244,000
Assets, Purchases	37,769,000	12,344,000
Assets, Sales	(40,710,000)	(27,745,000)
Assets, Transfers In and/or Out of Level 3	0	0
Assets, Ending Balance	32,645,000	5,778,000
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
Assets, Accreted Discounts/Amortized Premiums	563,000	8,000
Assets, Realized Gain (Loss)	2,000	0
Assets, Change In Net Unrealized Gain/(Loss)	(460,000)	159,000
Assets, Purchases	20,023,000	0
Assets, Sales	(10,000)	0
Assets, Transfers In and/or Out of Level 3	0	0
Assets, Ending Balance	44,005,000	9,713,000
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
Assets, Accreted Discounts/Amortized Premiums	0
Assets, Realized Gain (Loss)	18,000
Assets, Change In Net Unrealized Gain/(Loss)	(260,000)
Assets, Purchases	0
Assets, Sales	(254,000)
Assets, Transfers In and/or Out of Level 3	0
Assets, Ending Balance	23,566,000
Investments [Member] | Asset-backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	38,069,000	0
Assets, Accreted Discounts/Amortized Premiums	159,000	(49,000)
Assets, Realized Gain (Loss)	(97,000)	0
Assets, Change In Net Unrealized Gain/(Loss)	548,000	(107,000)
Assets, Purchases	13,887,000	11,794,000
Assets, Sales	(5,108,000)	0
Assets, Transfers In and/or Out of Level 3	0	0
Assets, Ending Balance	47,458,000	11,638,000
Investments [Member] | Residential Real Estate [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)	(4,000)
Assets, Purchases	101,000
Assets, Sales	0
Assets, Transfers In and/or Out of Level 3	0
Assets, Ending Balance	97,000
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
Assets, Accreted Discounts/Amortized Premiums	0	0
Assets, Realized Gain (Loss)	1,190,000	3,251,000
Assets, Change In Net Unrealized Gain/(Loss)	(1,210,000)	(5,599,000)
Assets, Purchases	39,000	49,000
Assets, Sales	(1,491,000)	(7,334,000)
Assets, Transfers In and/or Out of Level 3	0	0
Assets, Ending Balance	15,174,000	26,398,000
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
Assets, Accreted Discounts/Amortized Premiums	0	0
Assets, Realized Gain (Loss)	1,190,000	3,251,000
Assets, Change In Net Unrealized Gain/(Loss)	(1,210,000)	(5,599,000)
Assets, Purchases	39,000	49,000
Assets, Sales	(1,491,000)	(7,334,000)
Assets, Transfers In and/or Out of Level 3	0	0
Assets, Ending Balance	15,174,000	26,398,000
Financial Derivatives - Liabilities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liabilities, Begining Balance	(350,000)	(1,000)
Liabilities, Accreted Discounts/Amortized Premiums	0	0
Liabilities, Realized Gain/(Loss)	(19,000)	(28,000)
Liabilities, Change In Net Unrealized Gain/(Loss)	0	(1,000)
Liabilities, Purchases	0	0
Liabilities, Sales	19,000	28,000
Liabilities, Transfers In and/or Out of Level 3	0	0
Liabilities, Ending Balance	(350,000)	(2,000)
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)
Liabilities, Accreted Discounts/Amortized Premiums	0	0
Liabilities, Realized Gain/(Loss)	(19,000)	(28,000)
Liabilities, Change In Net Unrealized Gain/(Loss)	0	(1,000)
Liabilities, Purchases	0	0
Liabilities, Sales	19,000	28,000
Liabilities, Transfers In and/or Out of Level 3	0	0
Liabilities, Ending Balance	$ (350,000)	$ (2,000)