Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Assets
Fixed maturities available-for-sale	$ 18,188,344	$ 16,589,783
Short-term investments available-for-sale	266,332	332,764
Collateral under securities lending agreements	142,022	7,099
Total derivative instruments	32,418	32,766
Separate account assets	24,605,526	22,331,391
Liabilities
Payable under securities lending agreements	142,022	7,099
Separate account liabilities	24,605,526	22,331,391
Derivative instruments designated as hedges
Assets
Total derivative instruments	31,014	27,135
Derivative instruments not designated as hedges
Assets
Total derivative instruments	1,404	5,631
Derivative instruments not designated as hedges | Interest rate swaps
Assets
Total derivative instruments	1,062	4,687
Derivative instruments not designated as hedges | Interest rate swaptions
Assets
Total derivative instruments	342	944
U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale	2,834,074	2,315,671
Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale	2,018,505	2,071,170
Corporate debt securities
Assets
Fixed maturities available-for-sale	10,374,091	8,895,746
Asset-backed securities
Assets
Fixed maturities available-for-sale	1,861,139	1,951,444
Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale	425,585	591,542
Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale	662,955	747,797
Collateralized debt obligations
Assets
Fixed maturities available-for-sale	11,995	16,413
Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Short-term investments available-for-sale	19,459	45,869
Collateral under securities lending agreements	142,022	7,099
Collateral under derivative counterparty collateral agreements	54,400	11,985
Separate account assets	12,171,024	10,646,426
Total assets	12,386,905	10,711,379
Liabilities
Payable under securities lending agreements	142,022	7,099
Payable under derivative counterparty collateral agreements		11,985
Separate account liabilities	14	74
Total liabilities	142,036	19,158
Recurring basis | Significant other observable inputs (Level 2)
Assets
Fixed maturities available-for-sale	17,920,952	16,274,244
Fixed maturities held for trading	367,600	147,526
Short-term investments available-for-sale	246,873	286,895
Total derivative instruments	32,418	32,766
Separate account assets	12,434,502	11,568,489
Total assets	31,002,345	28,309,920
Liabilities
Total derivative instruments	88,158	14,014
Separate account liabilities	352,653	278,796
Total liabilities	440,811	292,810
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges | Interest rate swaps
Assets
Total derivative instruments	26,371	20,894
Liabilities
Total derivative instruments	1,649	1,011
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges | Cross-currency swaps
Assets
Total derivative instruments	4,643	6,241
Liabilities
Total derivative instruments	85,752
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges | Interest rate swaps
Assets
Total derivative instruments	1,062	4,687
Liabilities
Total derivative instruments	757	13,003
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges | Interest rate swaptions
Assets
Total derivative instruments	342	944
Recurring basis | Significant other observable inputs (Level 2) | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale	2,834,074	2,315,671
Fixed maturities held for trading	263,634	36,352
Recurring basis | Significant other observable inputs (Level 2) | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale	2,018,505	2,071,170
Recurring basis | Significant other observable inputs (Level 2) | Corporate debt securities
Assets
Fixed maturities available-for-sale	10,372,269	8,859,250
Fixed maturities held for trading	61,336	60,243
Recurring basis | Significant other observable inputs (Level 2) | Asset-backed securities
Assets
Fixed maturities available-for-sale	1,595,601	1,672,423
Fixed maturities held for trading	42,630	43,905
Recurring basis | Significant other observable inputs (Level 2) | Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale	425,585	591,542
Recurring basis | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale	662,955	747,797
Fixed maturities held for trading		7,026
Recurring basis | Significant other observable inputs (Level 2) | Collateralized debt obligations
Assets
Fixed maturities available-for-sale	11,963	16,391
Recurring basis | Significant unobservable inputs (Level 3)
Assets
Fixed maturities available-for-sale	267,392	315,539
Separate account assets		2,118
Total assets	267,392	317,657
Recurring basis | Significant unobservable inputs (Level 3) | Corporate debt securities
Assets
Fixed maturities available-for-sale	1,822	36,496
Recurring basis | Significant unobservable inputs (Level 3) | Asset-backed securities
Assets
Fixed maturities available-for-sale	265,538	279,021
Recurring basis | Significant unobservable inputs (Level 3) | Collateralized debt obligations
Assets
Fixed maturities available-for-sale	32	22
Recurring basis | Total
Assets
Fixed maturities available-for-sale	18,188,344	16,589,783
Fixed maturities held for trading	367,600	147,526
Short-term investments available-for-sale	266,332	332,764
Collateral under securities lending agreements	142,022	7,099
Collateral under derivative counterparty collateral agreements	54,400	11,985
Total derivative instruments	32,418	32,766
Separate account assets	24,605,526	22,217,033
Total assets	43,656,642	39,338,956
Liabilities
Payable under securities lending agreements	142,022	7,099
Payable under derivative counterparty collateral agreements		11,985
Total derivative instruments	88,158	14,014
Separate account liabilities	352,667	278,870
Total liabilities	582,847	311,968
Recurring basis | Total | Derivative instruments designated as hedges | Interest rate swaps
Assets
Total derivative instruments	26,371	20,894
Liabilities
Total derivative instruments	1,649	1,011
Recurring basis | Total | Derivative instruments designated as hedges | Cross-currency swaps
Assets
Total derivative instruments	4,643	6,241
Liabilities
Total derivative instruments	85,752
Recurring basis | Total | Derivative instruments not designated as hedges | Interest rate swaps
Assets
Total derivative instruments	1,062	4,687
Liabilities
Total derivative instruments	757	13,003
Recurring basis | Total | Derivative instruments not designated as hedges | Interest rate swaptions
Assets
Total derivative instruments	342	944
Recurring basis | Total | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale	2,834,074	2,315,671
Fixed maturities held for trading	263,634	36,352
Recurring basis | Total | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale	2,018,505	2,071,170
Recurring basis | Total | Corporate debt securities
Assets
Fixed maturities available-for-sale	10,374,091	8,895,746
Fixed maturities held for trading	61,336	60,243
Recurring basis | Total | Asset-backed securities
Assets
Fixed maturities available-for-sale	1,861,139	1,951,444
Fixed maturities held for trading	42,630	43,905
Recurring basis | Total | Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale	425,585	591,542
Recurring basis | Total | Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale	662,955	747,797
Fixed maturities held for trading		7,026
Recurring basis | Total | Collateralized debt obligations
Assets
Fixed maturities available-for-sale	$ 11,995	$ 16,413