Valuation (Fair Value Measurement Using Significant Unobservable Inputs) (Details) (Level 3 [Member], USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	$ 504,861,000	$ 443,539,000	$ 314,296,000
Assets, Accreted Discounts/Amortized Premiums	20,276,000	15,455,000	11,619,000
Assets, Realized Gain/(Loss)	22,223,000	28,433,000	32,402,000
Assets, Change in Net Unrealized Gain/(Loss)	72,094,000	(54,734,000)	(2,896,000)
Assets, Purchases	454,993,000	390,471,000	373,218,000
Assets, Sales	(474,533,000)	(318,303,000)	(285,100,000)
Assets, Transfers In and/or Out of Level 3	0	0	0
Assets, Ending Balance	599,914,000	504,861,000	443,539,000
Liabilities, Beginning Balance	0
Liabilities, Accreted Discounts/Amortized Premiums	(55,000)
Liabilities, Realized Gain/(Loss)	(79,000)
Liabilities, Change in Net Unrealized Gain/(Loss)	(24,000)
Liabilities Purchases/Payments	265,000
Liabilities, Sales/Issuance	(1,443,000)
Liabilities, Transfers In and/or Out of Level 3	0
Liabilities, Ending Balance	(1,336,000)	0
Change in net unrealized gain (loss) on investments	52,400,000	(39,600,000)	18,900,000
Change in net unrealized gain (loss) in financial derivative assets	(10,800,000)	(14,200,000)	(21,000,000)
Unrealized Gain (Loss) On Derivative Liabilities, Level 3, Held At End Of Period	(1,000)
Change in net unrealized gain (loss) in securitized debt	(20,000)
Derivative Financial Instruments, Liabilities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liabilities, Beginning Balance			(10,548,000)
Liabilities, Accreted Discounts/Amortized Premiums			0
Liabilities, Realized Gain/(Loss)			(1,657,000)
Liabilities, Change in Net Unrealized Gain/(Loss)			3,881,000
Liabilities, Payments			8,324,000
Liabilities, Issuances			0
Liabilities, Transfers In and/or Out of Level 3			0
Liabilities, Ending Balance			0
Securitized Debt [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liabilities, Beginning Balance	0
Liabilities, Accreted Discounts/Amortized Premiums	(55,000)
Liabilities, Realized Gain/(Loss)	0
Liabilities, Change in Net Unrealized Gain/(Loss)	(23,000)
Liabilities, Payments	265,000
Liabilities, Issuances	(1,522,000)
Liabilities, Transfers In and/or Out of Level 3	0
Liabilities, Ending Balance	(1,335,000)
Credit Default Swaps On Asset Backed Securities [Member] | Derivative Financial Instruments, Liabilities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liabilities, Beginning Balance			(10,548,000)
Liabilities, Accreted Discounts/Amortized Premiums			0
Liabilities, Realized Gain/(Loss)			(1,657,000)
Liabilities, Change in Net Unrealized Gain/(Loss)			3,881,000
Liabilities, Payments			8,324,000
Liabilities, Issuances			0
Liabilities, Transfers In and/or Out of Level 3			0
Liabilities, Ending Balance			0
Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	443,363,000	340,689,000	210,364,000
Assets, Accreted Discounts/Amortized Premiums	20,276,000	15,455,000	11,619,000
Assets, Realized Gain/(Loss)	23,387,000	18,464,000	22,933,000
Assets, Change in Net Unrealized Gain/(Loss)	68,876,000	(49,013,000)	20,096,000
Assets, Purchases	444,153,000	384,529,000	326,136,000
Assets, Sales	(436,172,000)	(266,761,000)	(250,459,000)
Assets, Transfers In and/or Out of Level 3	0	0	0
Assets, Ending Balance	563,883,000	443,363,000	340,689,000
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	5,337,000	0
Assets, Accreted Discounts/Amortized Premiums	(2,277,000)	(1,457,000)
Assets, Realized Gain/(Loss)	111,000	97,000
Assets, Change in Net Unrealized Gain/(Loss)	(568,000)	(2,078,000)
Assets, Purchases	5,201,000	9,930,000
Assets, Sales	(1,160,000)	(1,155,000)
Assets, Transfers In and/or Out of Level 3	0	0
Assets, Ending Balance	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	417,533,000	338,839,000	210,364,000
Assets, Accreted Discounts/Amortized Premiums	21,896,000	16,259,000	11,632,000
Assets, Realized Gain/(Loss)	15,657,000	17,246,000	22,933,000
Assets, Change in Net Unrealized Gain/(Loss)	69,286,000	(43,559,000)	20,083,000
Assets, Purchases	323,554,000	342,360,000	324,286,000
Assets, Sales	(319,560,000)	(253,612,000)	(250,459,000)
Assets, Transfers In and/or Out of Level 3	0	0	0
Assets, Ending Balance	528,366,000	417,533,000	338,839,000
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	16,093,000	1,850,000	0
Assets, Accreted Discounts/Amortized Premiums	570,000	539,000	(13,000)
Assets, Realized Gain/(Loss)	7,619,000	1,121,000	0
Assets, Change in Net Unrealized Gain/(Loss)	(255,000)	(2,987,000)	13,000
Assets, Purchases	110,752,000	27,564,000	1,850,000
Assets, Sales	(115,452,000)	(11,994,000)	0
Assets, Transfers In and/or Out of Level 3	0	0	0
Assets, Ending Balance	19,327,000	16,093,000	1,850,000
Investments [Member] | Commercial Mortgage Loans [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	4,400,000	0
Assets, Accreted Discounts/Amortized Premiums	87,000	114,000
Assets, Realized Gain/(Loss)	0	0
Assets, Change in Net Unrealized Gain/(Loss)	413,000	(389,000)
Assets, Purchases	4,646,000	4,675,000
Assets, Sales	0	0
Assets, Transfers In and/or Out of Level 3	0	0
Assets, Ending Balance	9,546,000	4,400,000
Financial Derivatives - Assets [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance	61,498,000	102,850,000	103,932,000
Assets, Accreted Discounts/Amortized Premiums	0	0	0
Assets, Realized Gain/(Loss)	(1,164,000)	9,969,000	9,469,000
Assets, Change in Net Unrealized Gain/(Loss)	3,218,000	(5,721,000)	(22,992,000)
Assets, Purchases	10,840,000	5,942,000	47,082,000
Assets, Sales	(38,361,000)	(51,542,000)	(34,641,000)
Assets, Transfers In and/or Out of Level 3	0	0	0
Assets, Ending Balance	36,031,000	61,498,000	102,850,000
Financial Derivatives - Assets [Member] | Credit Default Swaps On Corporate Bond [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance			8,476,000
Assets, Accreted Discounts/Amortized Premiums			0
Assets, Realized Gain/(Loss)			(2,281,000)
Assets, Change in Net Unrealized Gain/(Loss)			(2,650,000)
Assets, Purchases			1,713,000
Assets, Sales			(5,258,000)
Assets, Transfers In and/or Out of Level 3			0
Assets, Ending Balance			0
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	61,498,000	102,850,000	95,199,000
Assets, Accreted Discounts/Amortized Premiums	0	0	0
Assets, Realized Gain/(Loss)	(1,164,000)	9,969,000	11,415,000
Assets, Change in Net Unrealized Gain/(Loss)	3,218,000	(5,721,000)	(20,085,000)
Assets, Purchases	10,840,000	5,942,000	45,369,000
Assets, Sales	(38,361,000)	(51,542,000)	(29,048,000)
Assets, Transfers In and/or Out of Level 3	0	0	0
Assets, Ending Balance	36,031,000	61,498,000	102,850,000
Financial Derivatives - Assets [Member] | Other Swaps [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Assets, Beginning Balance			257,000
Assets, Accreted Discounts/Amortized Premiums			0
Assets, Realized Gain/(Loss)			335,000
Assets, Change in Net Unrealized Gain/(Loss)			(257,000)
Assets, Purchases			0
Assets, Sales			(335,000)
Assets, Transfers In and/or Out of Level 3			0
Assets, Ending Balance			0
Derivative Financial Instruments, Liabilities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liabilities, Beginning Balance	0
Liabilities, Accreted Discounts/Amortized Premiums	0
Liabilities, Realized Gain/(Loss)	(79,000)
Liabilities, Change in Net Unrealized Gain/(Loss)	(1,000)
Liabilities, Purchases	0
Liabilities, Sales	79,000
Liabilities, Transfers In and/or Out of Level 3	0
Liabilities, Ending Balance	(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, Beginning Balance	0
Liabilities, Accreted Discounts/Amortized Premiums	0
Liabilities, Realized Gain/(Loss)	(79,000)
Liabilities, Change in Net Unrealized Gain/(Loss)	(1,000)
Liabilities, Purchases	0
Liabilities, Sales	79,000
Liabilities, Transfers In and/or Out of Level 3	0
Liabilities, Ending Balance	$ (1,000)