Valuation (Fair Value Measurement Using Significant Unobservable Inputs) (Details) (Level 3 [Member], USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets, Beginning Balance	$ 461,540,000	$ 448,542,000	$ 504,861,000	$ 443,539,000
Assets, Accreted Discounts/Amortized Premiums	5,940,000	4,144,000	14,358,000	11,337,000
Assets, Realized Gain/(Loss)	12,419,000	6,157,000	14,299,000	27,016,000
Assets, Change in Net Unrealized Gain/(Loss)	19,562,000	(17,424,000)	53,924,000	(46,516,000)
Assets, Purchases	150,769,000	116,128,000	333,200,000	331,073,000
Assets, Sales	(103,172,000)	(76,662,000)	(373,584,000)	(285,564,000)
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	547,058,000	480,885,000	547,058,000	480,885,000
Liabilities, Beginning Balance	(1,415,000)		0
Liabilities, Accreted Discounts/Amortized Premiums	(15,000)		(44,000)
Liabilities, Realized Gain/(Loss)	0		0
Liabilities, Change in Net Unrealized Gain/(Loss)	(51,000)		(30,000)
Liabilities, Purchases	42,000		157,000
Liabilities, Sales	0		(1,522,000)
Liabilities, Transfers In and/or Out of Level 3	0		0
Liabilities, Ending Balance	(1,439,000)		(1,439,000)
Change in net unrealized gain (loss) on investments	23,300,000	(15,900,000)	33,700,000	(32,200,000)
Change in net unrealized gain (loss) in financial derivative assets	(3,800,000)	2,300,000	(8,100,000)	(12,600,000)
Change in net unrealized gain (loss) in securitized debt	(50,000)		(30,000)
Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets, Beginning Balance	422,781,000	378,713,000	443,363,000	340,689,000
Assets, Accreted Discounts/Amortized Premiums	5,940,000	4,144,000	14,358,000	11,337,000
Assets, Realized Gain/(Loss)	8,456,000	3,069,000	15,813,000	18,620,000
Assets, Change in Net Unrealized Gain/(Loss)	23,385,000	(17,176,000)	49,639,000	(41,529,000)
Assets, Purchases	150,664,000	116,006,000	332,870,000	330,547,000
Assets, Sales	(95,628,000)	(63,777,000)	(340,445,000)	(238,685,000)
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	515,598,000	420,979,000	515,598,000	420,979,000
Financial Derivatives - Assets [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets, Beginning Balance	38,759,000	69,829,000	61,498,000	102,850,000
Assets, Accreted Discounts/Amortized Premiums	0	0	0	0
Assets, Realized Gain/(Loss)	3,963,000	3,088,000	(1,514,000)	8,396,000
Assets, Change in Net Unrealized Gain/(Loss)	(3,823,000)	(248,000)	4,285,000	(4,987,000)
Assets, Purchases	105,000	122,000	330,000	526,000
Assets, Sales	(7,544,000)	(12,885,000)	(33,139,000)	(46,879,000)
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	31,460,000	59,906,000	31,460,000	59,906,000
Securitized Debt [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Liabilities, Beginning Balance	(1,415,000)		0
Liabilities, Accreted Discounts/Amortized Premiums	(15,000)		(44,000)
Liabilities, Realized Gain/(Loss)	0		0
Liabilities, Change in Net Unrealized Gain/(Loss)	(51,000)		(30,000)
Liabilities, Purchases	42,000		157,000
Liabilities, Sales	0		(1,522,000)
Liabilities, Transfers In and/or Out of Level 3	0		0
Liabilities, Ending Balance	(1,439,000)		(1,439,000)
Credit Default Swaps On Asset Backed Securities [Member] | Financial Derivatives - Assets [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets, Beginning Balance	38,759,000	69,829,000	61,498,000	102,850,000
Assets, Accreted Discounts/Amortized Premiums	0	0	0	0
Assets, Realized Gain/(Loss)	3,963,000	3,088,000	(1,514,000)	8,396,000
Assets, Change in Net Unrealized Gain/(Loss)	(3,823,000)	(248,000)	4,285,000	(4,987,000)
Assets, Purchases	105,000	122,000	330,000	526,000
Assets, Sales	(7,544,000)	(12,885,000)	(33,139,000)	(46,879,000)
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	31,460,000	59,906,000	31,460,000	59,906,000
U.S. Treasury And Agency Residential Mortgage-Backed Securities [Member] | Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets, Beginning Balance	4,667,000	5,227,000	5,337,000	0
Assets, Accreted Discounts/Amortized Premiums	(534,000)	(423,000)	(1,711,000)	(843,000)
Assets, Realized Gain/(Loss)	111,000	0	111,000	97,000
Assets, Change in Net Unrealized Gain/(Loss)	(314,000)	(2,169,000)	(679,000)	(2,320,000)
Assets, Purchases	1,057,000	2,826,000	1,929,000	9,682,000
Assets, Sales	(1,160,000)	0	(1,160,000)	(1,155,000)
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	3,827,000	5,461,000	3,827,000	5,461,000
Private Label Residential Mortgage-Backed Securities [Member] | Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets, Beginning Balance	385,059,000	357,894,000	417,533,000	338,839,000
Assets, Accreted Discounts/Amortized Premiums	6,182,000	4,389,000	15,471,000	11,710,000
Assets, Realized Gain/(Loss)	5,440,000	3,107,000	12,489,000	17,664,000
Assets, Change in Net Unrealized Gain/(Loss)	24,013,000	(12,994,000)	49,149,000	(35,632,000)
Assets, Purchases	92,412,000	106,262,000	251,030,000	292,521,000
Assets, Sales	(55,181,000)	(62,394,000)	(287,747,000)	(228,838,000)
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	457,925,000	396,264,000	457,925,000	396,264,000
Private Label Commercial Mortgage-Backed Securities [Member] | Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets, Beginning Balance	28,405,000	10,942,000	16,093,000	1,850,000
Assets, Accreted Discounts/Amortized Premiums	271,000	147,000	527,000	378,000
Assets, Realized Gain/(Loss)	2,905,000	(38,000)	3,213,000	859,000
Assets, Change in Net Unrealized Gain/(Loss)	(293,000)	(1,707,000)	990,000	(3,185,000)
Assets, Purchases	57,195,000	6,918,000	79,911,000	23,669,000
Assets, Sales	(39,287,000)	(1,383,000)	(51,538,000)	(8,692,000)
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	49,196,000	14,879,000	49,196,000	14,879,000
Commercial Mortgage Loans [Member] | Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets, Beginning Balance	4,650,000	4,650,000	4,400,000	0
Assets, Accreted Discounts/Amortized Premiums	21,000	31,000	71,000	92,000
Assets, Realized Gain/(Loss)	0	0	0	0
Assets, Change in Net Unrealized Gain/(Loss)	(21,000)	(306,000)	179,000	(392,000)
Assets, Purchases	0	0	0	4,675,000
Assets, Sales	0	0	0	0
Assets, Transfers In and/or Out of Level 3	0	0	0	0
Assets, Ending Balance	$ 4,650,000	$ 4,375,000	$ 4,650,000	$ 4,375,000