Investments (Details) - Contractual maturities of available for sale securities and held to maturity income securities (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
AFS securities
Due less than one year, amortized cost	$ 2,165,727	$ 2,004,395
Due less than one year, fair value	2,176,654	2,020,361
Due after 1 through 5 years, amortized cost	8,413,665	7,736,717
Due after 1 through 5 years, fair value	8,787,679	7,909,354
Due after 5 through 10 years, amortized cost	4,252,492	3,619,141
Due after 5 through 10 years, fair value	4,609,302	3,777,073
Due after 10 years, amortized cost	2,309,841	3,257,886
Due after 10 years, fair value	2,663,911	3,488,330
Subtotal - amortized cost	17,141,725	16,618,139
Subtotal - fair value	18,237,546	17,195,118
Mortgage and asset-backed securities - amortized cost	9,008,795	9,153,576
Mortgage and asset-backed securities - fair value	9,188,747	8,994,907
Amortized cost of Fixed maturities	26,150,520	25,771,715
Fixed maturities, at fair value	27,426,293	26,190,025
Fixed maturities - HTM
Due less than one year, amortized cost	25,392	11,796
Due less than one year, fair value	25,615	11,768
Due after 1 through 5 years, amortized cost	171,077	122,091
Due after 1 through 5 years, fair value	180,080	123,871
Due after 5 through 10 years, amortized cost	407,520	393,865
Due after 5 through 10 years, fair value	447,813	402,424
Due after 10 years, amortized cost	1,880,486	1,771,530
Due after 10 years, fair value	2,207,457	1,960,886
Subtotal - amortized cost	2,484,475	2,299,282
Subtotal - fair value	2,860,965	2,498,949
Mortage and asset-backed securities - amortized cost	321,321	369,696
Mortgage and asset-backed securities - fair value	361,935	396,739
Amortized Cost	2,805,796	2,668,978
Fair Value	3,222,900	2,895,688
RMBS - Agency
AFS securities
Mortgage and asset-backed securities - amortized cost	5,126,903	5,189,473
Mortgage and asset-backed securities - fair value	5,376,903	5,379,406
Fixed maturities, at fair value	5,376,903	5,379,406
RMBS - Non-Agency
AFS securities
Mortgage and asset-backed securities - amortized cost	705,785	851,557
Mortgage and asset-backed securities - fair value	636,325	641,815
Fixed maturities, at fair value	636,325	641,815
Fixed maturities - HTM
Mortage and asset-backed securities - amortized cost	83,896	80,955
Mortgage and asset-backed securities - fair value	93,478	87,443
Amortized Cost	83,896	80,955
Fair Value	93,478	87,443
CMBS
AFS securities
Mortgage and asset-backed securities - amortized cost	963,365	927,684
Mortgage and asset-backed securities - fair value	1,034,409	974,835
Fixed maturities, at fair value	1,034,409	974,835
Fixed maturities - HTM
Mortage and asset-backed securities - amortized cost	12,917	0
Mortgage and asset-backed securities - fair value	15,026	0
Amortized Cost	12,917
Fair Value	15,026
CDO
AFS securities
Mortgage and asset-backed securities - amortized cost	791,737	843,553
Mortgage and asset-backed securities - fair value	682,566	658,602
Fixed maturities, at fair value	682,566	658,602
Other asset-backed securities
AFS securities
Mortgage and asset-backed securities - amortized cost	1,421,005	1,341,309
Mortgage and asset-backed securities - fair value	1,458,544	1,340,249
Fixed maturities, at fair value	1,458,544	1,340,249
Fixed maturities - HTM
Mortage and asset-backed securities - amortized cost	224,508	288,741
Mortgage and asset-backed securities - fair value	253,431	309,296
Amortized Cost	224,508	288,741
Fair Value	$ 253,431	$ 309,296