Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Assets
Fixed maturities available-for-sale	$ 18,209,545	$ 18,188,344
Short-term investments available-for-sale	1,997,680	266,332
Collateral under securities lending agreements	64,116	142,022
Collateral under derivative counterparty collateral agreements	222
Total derivative instruments	32,690	26,647
Separate account assets	25,369,168	24,605,526
Liabilities
Payable under securities lending agreements	64,116	142,022
Separate account liabilities	25,369,168	24,605,526
Derivative instruments designated as hedges
Assets
Total derivative instruments	29,511	31,014
Derivative instruments not designated as hedges
Assets
Total derivative instruments	1,598	1,404
Derivative instruments not designated as hedges | Interest rate swaps
Assets
Total derivative instruments	749	1,062
Derivative instruments not designated as hedges | Interest rate swaptions
Assets
Total derivative instruments	849	342
U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale	2,900,470	2,834,074
Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale	1,959,093	2,018,505
Foreign government securities
Assets
Fixed maturities available-for-sale	2,673
Corporate debt securities
Assets
Fixed maturities available-for-sale	10,543,055	10,374,091
Asset-backed securities
Assets
Fixed maturities available-for-sale	1,830,603	1,861,139
Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale	347,680	425,585
Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale	613,389	662,955
Collateralized debt obligations
Assets
Fixed maturities available-for-sale	12,582	11,995
Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Short-term investments available-for-sale	189,222	19,459
Collateral under securities lending agreements	64,116	142,022
Collateral under derivative counterparty collateral agreements	83,160	54,400
Separate account assets	13,123,478	12,171,024
Total assets	13,459,976	12,386,905
Liabilities
Payable under securities lending agreements	64,116	142,022
Separate account liabilities	34	14
Total liabilities	64,150	142,036
Recurring basis | Significant other observable inputs (Level 2)
Assets
Fixed maturities available-for-sale	17,946,949	17,920,952
Fixed maturities held for trading	198,926	367,600
Short-term investments available-for-sale	1,808,458	246,873
Total derivative instruments	31,109	32,418
Separate account assets	12,245,690	12,434,502
Total assets	32,231,132	31,002,345
Liabilities
Total derivative instruments	111,491	88,158
Separate account liabilities	188,857	352,653
Total liabilities	300,348	440,811
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges | Interest rate swaps
Assets
Total derivative instruments	23,501	26,371
Liabilities
Total derivative instruments	141	1,649
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges | Cross-currency swaps
Assets
Total derivative instruments	6,010	4,643
Liabilities
Total derivative instruments	109,591	85,752
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges | Interest rate swaps
Assets
Total derivative instruments	749	1,062
Liabilities
Total derivative instruments	1,759	757
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges | Interest rate swaptions
Assets
Total derivative instruments	849	342
Recurring basis | Significant other observable inputs (Level 2) | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale	2,900,470	2,834,074
Fixed maturities held for trading	98,397	263,634
Recurring basis | Significant other observable inputs (Level 2) | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale	1,959,093	2,018,505
Recurring basis | Significant other observable inputs (Level 2) | Foreign government securities
Assets
Fixed maturities available-for-sale	2,673
Recurring basis | Significant other observable inputs (Level 2) | Corporate debt securities
Assets
Fixed maturities available-for-sale	10,541,259	10,372,269
Fixed maturities held for trading	58,842	61,336
Recurring basis | Significant other observable inputs (Level 2) | Asset-backed securities
Assets
Fixed maturities available-for-sale	1,569,835	1,595,601
Fixed maturities held for trading	41,687	42,630
Recurring basis | Significant other observable inputs (Level 2) | Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale	347,680	425,585
Recurring basis | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale	613,389	662,955
Recurring basis | Significant other observable inputs (Level 2) | Collateralized debt obligations
Assets
Fixed maturities available-for-sale	12,550	11,963
Recurring basis | Significant unobservable inputs (Level 3)
Assets
Fixed maturities available-for-sale	262,596	267,392
Total assets	262,596	267,392
Recurring basis | Significant unobservable inputs (Level 3) | Corporate debt securities
Assets
Fixed maturities available-for-sale	1,796	1,822
Recurring basis | Significant unobservable inputs (Level 3) | Asset-backed securities
Assets
Fixed maturities available-for-sale	260,768	265,538
Recurring basis | Significant unobservable inputs (Level 3) | Collateralized debt obligations
Assets
Fixed maturities available-for-sale	32	32
Recurring basis | Total
Assets
Fixed maturities available-for-sale	18,209,545	18,188,344
Fixed maturities held for trading	198,926	367,600
Short-term investments available-for-sale	1,997,680	266,332
Collateral under securities lending agreements	64,116	142,022
Collateral under derivative counterparty collateral agreements	83,160	54,400
Total derivative instruments	31,109	32,418
Separate account assets	25,369,168	24,605,526
Total assets	45,953,704	43,656,642
Liabilities
Payable under securities lending agreements	64,116	142,022
Total derivative instruments	111,491	88,158
Separate account liabilities	188,891	352,667
Total liabilities	364,498	582,847
Recurring basis | Total | Derivative instruments designated as hedges | Interest rate swaps
Assets
Total derivative instruments	23,501	26,371
Liabilities
Total derivative instruments	141	1,649
Recurring basis | Total | Derivative instruments designated as hedges | Cross-currency swaps
Assets
Total derivative instruments	6,010	4,643
Liabilities
Total derivative instruments	109,591	85,752
Recurring basis | Total | Derivative instruments not designated as hedges | Interest rate swaps
Assets
Total derivative instruments	749	1,062
Liabilities
Total derivative instruments	1,759	757
Recurring basis | Total | Derivative instruments not designated as hedges | Interest rate swaptions
Assets
Total derivative instruments	849	342
Recurring basis | Total | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale	2,900,470	2,834,074
Fixed maturities held for trading	98,397	263,634
Recurring basis | Total | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale	1,959,093	2,018,505
Recurring basis | Total | Foreign government securities
Assets
Fixed maturities available-for-sale	2,673
Recurring basis | Total | Corporate debt securities
Assets
Fixed maturities available-for-sale	10,543,055	10,374,091
Fixed maturities held for trading	58,842	61,336
Recurring basis | Total | Asset-backed securities
Assets
Fixed maturities available-for-sale	1,830,603	1,861,139
Fixed maturities held for trading	41,687	42,630
Recurring basis | Total | Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale	347,680	425,585
Recurring basis | Total | Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale	613,389	662,955
Recurring basis | Total | Collateralized debt obligations
Assets
Fixed maturities available-for-sale	$ 12,582	$ 11,995