Valuation (Fair Value Measurement Using Significant Unobservable Inputs) (Details) (Level 3 [Member], USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	$ 599,914,000	$ 504,861,000
Assets, Accreted Discounts/Amortized Premiums	5,786,000	3,895,000
Assets, Realized Gain (Loss)	17,404,000	1,801,000
Assets, Change In Net Unrealized Gain/(Loss)	21,955,000	26,650,000
Assets, Purchases	107,235,000	100,981,000
Assets, Sales	(124,719,000)	(158,525,000)
Assets, Transfers In and/or Out of Level 3	0	0
Assets, Ending Balance	627,575,000	479,663,000
Liabilities, Begining Balance	(1,336,000)
Liabilities, Accreted Discounts/Amortized Premiums	(9,000)
Liabilities, Realized Gain/(Loss)	(28,000)
Liabilities, Change In Net Unrealized Gain/(Loss)	68,000
Liabilities Purchases/Payments	70,000
Liabilities, Sales/Issuance	28,000
Liabilities, Transfers In and/or Out of Level 3	0
Liabilities, Ending Balance	(1,207,000)
Change in net unrealized gain (loss) on investments	28,300,000	12,700,000
Change in net unrealized gain (loss) in financial derivative assets	(7,200,000)	(4,500,000)
Change in net unrealized Gain (Loss) on financial derivative liabilities	(1,000)
Change in net unrealized gain (loss) in securitized debt	70,000	10,000
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	(9,000)	(13,000)
Liabilities, Realized Gain/(Loss)	0	0
Liabilities, Change In Net Unrealized Gain/(Loss)	69,000	10,000
Liabilities, Payments	70,000	0
Liabilities, Issuances	0	(1,482,000)
Liabilities, Transfers In and/or Out of Level 3	0	0
Liabilities, Ending Balance	(1,205,000)	(1,485,000)
Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	563,883,000	443,363,000
Assets, Accreted Discounts/Amortized Premiums	5,786,000	3,895,000
Assets, Realized Gain (Loss)	14,153,000	6,545,000
Assets, Change In Net Unrealized Gain/(Loss)	27,554,000	19,851,000
Assets, Purchases	107,186,000	100,858,000
Assets, Sales	(117,385,000)	(143,595,000)
Assets, Transfers In and/or Out of Level 3	0	0
Assets, Ending Balance	601,177,000	430,917,000
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	6,644,000	5,337,000
Assets, Accreted Discounts/Amortized Premiums	(870,000)	(624,000)
Assets, Realized Gain (Loss)	0	0
Assets, Change In Net Unrealized Gain/(Loss)	238,000	431,000
Assets, Purchases	10,216,000	872,000
Assets, Sales	0	0
Assets, Transfers In and/or Out of Level 3	0	0
Assets, Ending Balance	16,228,000	6,016,000
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
Assets, Accreted Discounts/Amortized Premiums	6,640,000	4,374,000
Assets, Realized Gain (Loss)	15,602,000	6,201,000
Assets, Change In Net Unrealized Gain/(Loss)	24,020,000	17,660,000
Assets, Purchases	72,832,000	98,678,000
Assets, Sales	(89,640,000)	(136,216,000)
Assets, Transfers In and/or Out of Level 3	0	0
Assets, Ending Balance	557,820,000	408,230,000
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
Assets, Accreted Discounts/Amortized Premiums	57,000	117,000
Assets, Realized Gain (Loss)	(1,449,000)	344,000
Assets, Change In Net Unrealized Gain/(Loss)	3,244,000	1,688,000
Assets, Purchases	12,344,000	1,308,000
Assets, Sales	(27,745,000)	(7,379,000)
Assets, Transfers In and/or Out of Level 3	0	0
Assets, Ending Balance	5,778,000	12,171,000
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
Assets, Accreted Discounts/Amortized Premiums	8,000	28,000
Assets, Realized Gain (Loss)	0	0
Assets, Change In Net Unrealized Gain/(Loss)	159,000	72,000
Assets, Purchases	0	0
Assets, Sales	0	0
Assets, Transfers In and/or Out of Level 3	0	0
Assets, Ending Balance	9,713,000	4,500,000
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	(49,000)
Assets, Realized Gain (Loss)	0
Assets, Change In Net Unrealized Gain/(Loss)	(107,000)
Assets, Purchases	11,794,000
Assets, Sales	0
Assets, Transfers In and/or Out of Level 3	0
Assets, Ending Balance	11,638,000
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
Assets, Accreted Discounts/Amortized Premiums	0	0
Assets, Realized Gain (Loss)	3,251,000	(4,744,000)
Assets, Change In Net Unrealized Gain/(Loss)	(5,599,000)	6,799,000
Assets, Purchases	49,000	123,000
Assets, Sales	(7,334,000)	(14,930,000)
Assets, Transfers In and/or Out of Level 3	0	0
Assets, Ending Balance	26,398,000	48,746,000
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
Assets, Accreted Discounts/Amortized Premiums	0	0
Assets, Realized Gain (Loss)	3,251,000	(4,744,000)
Assets, Change In Net Unrealized Gain/(Loss)	(5,599,000)	6,799,000
Assets, Purchases	49,000	123,000
Assets, Sales	(7,334,000)	(14,930,000)
Assets, Transfers In and/or Out of Level 3	0	0
Assets, Ending Balance	26,398,000	48,746,000
Derivative Financial Instruments, Liabilities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liabilities, Begining Balance	(1,000)
Liabilities, Accreted Discounts/Amortized Premiums	0
Liabilities, Realized Gain/(Loss)	(28,000)
Liabilities, Change In Net Unrealized Gain/(Loss)	(1,000)
Liabilities, Purchases	0
Liabilities, Sales	28,000
Liabilities, Transfers In and/or Out of Level 3	0
Liabilities, Ending Balance	(2,000)
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)
Liabilities, Accreted Discounts/Amortized Premiums	0
Liabilities, Realized Gain/(Loss)	(28,000)
Liabilities, Change In Net Unrealized Gain/(Loss)	(1,000)
Liabilities, Purchases	0
Liabilities, Sales	28,000
Liabilities, Transfers In and/or Out of Level 3	0
Liabilities, Ending Balance	$ (2,000)