Investments (Details) - Contractual maturities of available for sale securities and held to maturity income securities (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
AFS securities
Due less than one year, amortized cost	$ 1,939,803	$ 2,004,395
Due less than one year, fair value	1,952,250	2,020,361
Due after 1 through 5 years, amortized cost	8,521,090	7,736,717
Due after 1 through 5 years, fair value	8,877,512	7,909,354
Due after 5 through 10 years, amortized cost	4,701,391	3,619,141
Due after 5 through 10 years, fair value	5,065,158	3,777,073
Due after 10 years, amortized cost	2,427,680	3,257,886
Due after 10 years, fair value	2,790,996	3,488,330
Subtotal - amortized cost	17,589,964	16,618,139
Subtotal - fair value	18,685,916	17,195,118
Mortgage and asset-backed securities - amortized cost	8,954,842	9,153,576
Mortgage and asset-backed securities - fair value	9,161,004	8,994,907
Amortized cost of Fixed maturities	26,544,806	25,771,715
Fixed maturities, at fair value	27,846,920	26,190,025
Fixed maturities - HTM
Due less than one year, amortized cost	36,515	11,796
Due less than one year, fair value	37,580	11,768
Due after 1 through 5 years, amortized cost	195,121	122,091
Due after 1 through 5 years, fair value	205,562	123,871
Due after 5 through 10 years, amortized cost	377,541	393,865
Due after 5 through 10 years, fair value	420,008	402,424
Due after 10 years, amortized cost	1,886,974	1,771,530
Due after 10 years, fair value	2,239,688	1,960,886
Subtotal - amortized cost	2,496,151	2,299,282
Subtotal - fair value	2,902,838	2,498,949
Mortage and asset-backed securities - amortized cost	318,296	369,696
Mortgage and asset-backed securities - fair value	359,966	396,739
Amortized Cost	2,814,447	2,668,978
Fair Value	3,262,804	2,895,688
RMBS - Agency
AFS securities
Mortgage and asset-backed securities - amortized cost	5,054,097	5,189,473
Mortgage and asset-backed securities - fair value	5,255,493	5,379,406
Fixed maturities, at fair value	5,255,493	5,379,406
RMBS - Non-Agency
AFS securities
Mortgage and asset-backed securities - amortized cost	678,469	851,557
Mortgage and asset-backed securities - fair value	647,733	641,815
Fixed maturities, at fair value	647,733	641,815
Fixed maturities - HTM
Mortage and asset-backed securities - amortized cost	83,205	80,955
Mortgage and asset-backed securities - fair value	93,707	87,443
Amortized Cost	83,205	80,955
Fair Value	93,707	87,443
CMBS
AFS securities
Mortgage and asset-backed securities - amortized cost	1,010,794	927,684
Mortgage and asset-backed securities - fair value	1,077,251	974,835
Fixed maturities, at fair value	1,077,251	974,835
Fixed maturities - HTM
Mortage and asset-backed securities - amortized cost	12,751	0
Mortgage and asset-backed securities - fair value	14,799	0
Amortized Cost	12,751
Fair Value	14,799
CDO
AFS securities
Mortgage and asset-backed securities - amortized cost	784,999	843,553
Mortgage and asset-backed securities - fair value	709,816	658,602
Fixed maturities, at fair value	709,816	658,602
Other asset-backed securities
AFS securities
Mortgage and asset-backed securities - amortized cost	1,426,483	1,341,309
Mortgage and asset-backed securities - fair value	1,470,711	1,340,249
Fixed maturities, at fair value	1,470,711	1,340,249
Fixed maturities - HTM
Mortage and asset-backed securities - amortized cost	222,340	288,741
Mortgage and asset-backed securities - fair value	251,460	309,296
Amortized Cost	222,340	288,741
Fair Value	$ 251,460	$ 309,296