Investments (Details) - Contractual maturities of available for sale securities and held to maturity income securities (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
AFS securities
Due less than one year, amortized cost	$ 1,983,583	$ 2,004,395
Due less than one year, fair value	1,999,239	2,020,361
Due after 1 through 5 years, amortized cost	7,976,631	7,736,717
Due after 1 through 5 years, fair value	8,243,009	7,909,354
Due after 5 through 10 years, amortized cost	3,401,824	3,619,141
Due after 5 through 10 years, fair value	3,610,915	3,777,073
Due after 10 years, amortized cost	3,320,524	3,257,886
Due after 10 years, fair value	3,640,235	3,488,330
Subtotal - amortized cost	16,682,562	16,618,139
Subtotal - fair value	17,493,398	17,195,118
Mortgage and asset-backed securities - amortized cost	8,760,217	9,153,576
Mortgage and asset-backed securities - fair value	8,749,404	8,994,907
Amortized cost of Fixed maturities	25,442,779	25,771,715
Fixed maturities, at fair value	26,242,802	26,190,025
Fixed maturities - HTM
Due less than one year, amortized cost	30,847	11,796
Due less than one year, fair value	31,138	11,768
Due after 1 through 5 years, amortized cost	165,678	122,091
Due after 1 through 5 years, fair value	172,104	123,871
Due after 5 through 10 years, amortized cost	365,103	393,865
Due after 5 through 10 years, fair value	386,251	402,424
Due after 10 years, amortized cost	1,843,808	1,771,530
Due after 10 years, fair value	2,079,157	1,960,886
Subtotal - amortized cost	2,405,436	2,299,282
Subtotal - fair value	2,668,650	2,498,949
Mortage and asset-backed securities - amortized cost	310,921	369,696
Mortgage and asset-backed securities - fair value	338,342	396,739
Amortized Cost	2,716,357	2,668,978
Fair Value	3,006,992	2,895,688
RMBS - Agency
AFS securities
Mortgage and asset-backed securities - amortized cost	5,001,023	5,189,473
Mortgage and asset-backed securities - fair value	5,202,313	5,379,406
Amortized cost of Fixed maturities	5,001,023	5,189,473
Fixed maturities, at fair value	5,202,313	5,379,406
RMBS - Non-Agency
AFS securities
Mortgage and asset-backed securities - amortized cost	728,309	851,557
Mortgage and asset-backed securities - fair value	584,543	641,815
Amortized cost of Fixed maturities	728,309	851,557
Fixed maturities, at fair value	584,543	641,815
Fixed maturities - HTM
Mortage and asset-backed securities - amortized cost	81,028	80,955
Mortgage and asset-backed securities - fair value	87,090	87,443
Amortized Cost	81,028	80,955
Fair Value	87,090	87,443
CMBS
AFS securities
Mortgage and asset-backed securities - amortized cost	842,802	927,684
Mortgage and asset-backed securities - fair value	904,792	974,835
Amortized cost of Fixed maturities	842,802	927,684
Fixed maturities, at fair value	904,792	974,835
Fixed maturities - HTM
Mortage and asset-backed securities - amortized cost	12,546	0
Mortgage and asset-backed securities - fair value	14,213	0
Amortized Cost	12,546
Fair Value	14,213
CDO
AFS securities
Mortgage and asset-backed securities - amortized cost	796,018	843,553
Mortgage and asset-backed securities - fair value	650,018	658,602
Amortized cost of Fixed maturities	796,018	843,553
Fixed maturities, at fair value	650,018	658,602
Other asset-backed securities
AFS securities
Mortgage and asset-backed securities - amortized cost	1,392,065	1,341,309
Mortgage and asset-backed securities - fair value	1,407,738	1,340,249
Amortized cost of Fixed maturities	1,392,065	1,341,309
Fixed maturities, at fair value	1,407,738	1,340,249
Fixed maturities - HTM
Mortage and asset-backed securities - amortized cost	217,347	288,741
Mortgage and asset-backed securities - fair value	237,039	309,296
Amortized Cost	217,347	288,741
Fair Value	$ 237,039	$ 309,296