Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Assets
Fixed maturities available-for-sale	$ 18,691,185	$ 18,188,344
Short-term investments available-for-sale	1,925,769	266,332
Collateral under securities lending agreements	45,813	142,022
Total derivative instruments	25,837	26,647
Separate account assets	25,421,394	24,605,526
Liabilities
Payable under securities lending agreements	45,813	142,022
Separate account liabilities	25,421,394	24,605,526
Derivative instruments designated as hedges
Assets
Total derivative instruments	33,839	31,014
Derivative instruments not designated as hedges
Assets
Total derivative instruments	981	1,404
Derivative instruments not designated as hedges | Interest rate swaps
Assets
Total derivative instruments	463	1,062
Derivative instruments not designated as hedges | Interest rate swaptions
Assets
Total derivative instruments	518	342
U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale	2,991,545	2,834,074
Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale	2,066,010	2,018,505
Foreign government securities
Assets
Fixed maturities available-for-sale	2,744
Corporate debt securities
Assets
Fixed maturities available-for-sale	10,730,384	10,374,091
Asset-backed securities
Assets
Fixed maturities available-for-sale	1,830,469	1,861,139
Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale	402,712	425,585
Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale	654,066	662,955
Collateralized debt obligations
Assets
Fixed maturities available-for-sale	13,255	11,995
Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Short-term investments available-for-sale	247,083	19,459
Collateral under securities lending agreements	45,813	142,022
Collateral under derivative counterparty collateral agreements	74,780	54,400
Separate account assets	12,986,386	12,171,024
Total assets	13,354,062	12,386,905
Liabilities
Payable under securities lending agreements	45,813	142,022
Separate account liabilities	20	14
Total liabilities	45,833	142,036
Recurring basis | Significant other observable inputs (Level 2)
Assets
Fixed maturities available-for-sale	18,427,849	17,920,952
Fixed maturities held for trading	174,763	367,600
Short-term investments available-for-sale	1,678,686	246,873
Total derivative instruments	34,820	32,418
Separate account assets	12,435,008	12,434,502
Total assets	32,751,126	31,002,345
Liabilities
Total derivative instruments	107,448	88,158
Separate account liabilities	122,534	352,653
Total liabilities	229,982	440,811
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges | Interest rate swaps
Assets
Total derivative instruments	26,117	26,371
Liabilities
Total derivative instruments	820	1,649
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges | Cross-currency swaps
Assets
Total derivative instruments	7,722	4,643
Liabilities
Total derivative instruments	105,955
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges | Interest rate swaps
Assets
Total derivative instruments	463	1,062
Liabilities
Total derivative instruments	673	757
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges | Interest rate swaptions
Assets
Total derivative instruments	518	342
Recurring basis | Significant other observable inputs (Level 2) | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale	2,991,545	2,834,074
Fixed maturities held for trading	71,479	263,634
Recurring basis | Significant other observable inputs (Level 2) | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale	2,066,010	2,018,505
Recurring basis | Significant other observable inputs (Level 2) | Foreign government securities
Assets
Fixed maturities available-for-sale	2,744
Recurring basis | Significant other observable inputs (Level 2) | Corporate debt securities
Assets
Fixed maturities available-for-sale	10,728,583	10,372,269
Fixed maturities held for trading	61,089	61,336
Recurring basis | Significant other observable inputs (Level 2) | Asset-backed securities
Assets
Fixed maturities available-for-sale	1,568,966	1,595,601
Fixed maturities held for trading	42,195	42,630
Recurring basis | Significant other observable inputs (Level 2) | Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale	402,712	425,585
Recurring basis | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale	654,066	662,955
Recurring basis | Significant other observable inputs (Level 2) | Collateralized debt obligations
Assets
Fixed maturities available-for-sale	13,223	11,963
Recurring basis | Significant unobservable inputs (Level 3)
Assets
Fixed maturities available-for-sale	263,336	267,392
Total assets	263,336	267,392
Recurring basis | Significant unobservable inputs (Level 3) | Corporate debt securities
Assets
Fixed maturities available-for-sale	1,801	1,822
Recurring basis | Significant unobservable inputs (Level 3) | Asset-backed securities
Assets
Fixed maturities available-for-sale	261,503	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	18,691,185	18,188,344
Fixed maturities held for trading	174,763	367,600
Short-term investments available-for-sale	1,925,769	266,332
Collateral under securities lending agreements	45,813	142,022
Collateral under derivative counterparty collateral agreements	74,780	54,400
Total derivative instruments	34,820	32,418
Separate account assets	25,421,394	24,605,526
Total assets	46,368,524	43,656,642
Liabilities
Payable under securities lending agreements	45,813	142,022
Total derivative instruments	107,448	88,158
Separate account liabilities	122,554	352,667
Total liabilities	275,815	582,847
Recurring basis | Total | Derivative instruments designated as hedges | Interest rate swaps
Assets
Total derivative instruments	26,117	26,371
Liabilities
Total derivative instruments	820	1,649
Recurring basis | Total | Derivative instruments designated as hedges | Cross-currency swaps
Assets
Total derivative instruments	7,722	4,643
Liabilities
Total derivative instruments	105,955	85,752
Recurring basis | Total | Derivative instruments not designated as hedges | Interest rate swaps
Assets
Total derivative instruments	463	1,062
Liabilities
Total derivative instruments	673	757
Recurring basis | Total | Derivative instruments not designated as hedges | Interest rate swaptions
Assets
Total derivative instruments	518	342
Recurring basis | Total | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale	2,991,545	2,834,074
Fixed maturities held for trading	71,479	263,634
Recurring basis | Total | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale	2,066,010	2,018,505
Recurring basis | Total | Foreign government securities
Assets
Fixed maturities available-for-sale	2,744
Recurring basis | Total | Corporate debt securities
Assets
Fixed maturities available-for-sale	10,730,384	10,374,091
Fixed maturities held for trading	61,089	61,336
Recurring basis | Total | Asset-backed securities
Assets
Fixed maturities available-for-sale	1,830,469	1,861,139
Fixed maturities held for trading	42,195	42,630
Recurring basis | Total | Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale	402,712	425,585
Recurring basis | Total | Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale	654,066	662,955
Recurring basis | Total | Collateralized debt obligations
Assets
Fixed maturities available-for-sale	$ 13,255	$ 11,995