Fair Value Measurements - Fair Value Measurements - Recurring by FV hierarchy (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Assets
Fixed maturities available-for-sale:	$ 23,012,869	$ 22,153,703
Fixed maturities held-for-trading:	60,328	514,738
Short-term investments	823,071	303,988
Collateral under securities lending agreements	74,795	0
Collateral under derivative counterparty collateral agreements	13,006	0
Total derivative instruments	98,777	146,385
Separate account assets	27,866,779	27,037,765
Liabilities
Collateral under derivative counterparty collateral agreements	14,263	92,756
Total derivative instruments	82,940	38,281
Separate account liabilities	27,866,779	27,037,765
Derivative instruments designated as hedges:
Assets
Total derivative instruments	60,356	87,031
Liabilities
Total derivative instruments	35,184	8,294
Derivative instruments not designated as hedges:
Assets
Total derivative instruments	38,421	59,354
Liabilities
Total derivative instruments	47,756	29,987
Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	10,147	8,982
Liabilities
Total derivative instruments	12,624	13,340
Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments	156	354
Liabilities
Total derivative instruments	0	0
Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments	925
Liabilities
Total derivative instruments	4,076
Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	27,193	50,018
Liabilities
Total derivative instruments	31,056	16,647
U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale:	1,543,460	3,035,112
Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:	2,110,818	2,098,662
Corporate debt securities
Assets
Fixed maturities available-for-sale:	15,495,200	13,979,749
Asset-backed securities
Assets
Fixed maturities available-for-sale:	1,676,750	1,312,379
Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:	70,919	140,992
Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:	1,386,952	1,225,282
Collateralized debt obligations
Assets
Fixed maturities available-for-sale:	728,770	361,527
Recurring basis
Assets
Fixed maturities available-for-sale:	23,012,869	22,153,703
Fixed maturities held-for-trading:	60,328	514,738
Short-term investments	823,071	303,988
Collateral under securities lending agreements	74,795
Collateral under derivative counterparty collateral agreements	41,524	103,214
Total derivative instruments	98,777	146,385
Separate account assets	27,866,779	27,037,765
Total assets	51,978,143	50,259,793
Separate account assets excluded	397,000	430,000
Liabilities
Collateral under derivative counterparty collateral agreements		103,214
Total derivative instruments	82,940	38,281
Embedded derivatives - GLWB	10,221	5,712
Separate account liabilities	521,741	336,523
Total liabilities	614,902	483,730
Recurring basis | Derivative instruments designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	29,482	33,390
Recurring basis | Derivative instruments designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	30,874	53,641
Liabilities
Total derivative instruments	35,184	8,294
Recurring basis | Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	10,147	8,982
Liabilities
Total derivative instruments	12,624	13,340
Recurring basis | Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments	156	354
Recurring basis | Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments	925
Liabilities
Total derivative instruments	4,076
Recurring basis | Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	27,193	50,018
Liabilities
Total derivative instruments	31,056	16,647
Recurring basis | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale:	1,543,460	3,035,112
Fixed maturities held-for-trading:	20,291	458,067
Recurring basis | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:	2,110,818	2,098,662
Recurring basis | Corporate debt securities
Assets
Fixed maturities available-for-sale:	15,495,200	13,979,749
Fixed maturities held-for-trading:	38,956	55,591
Recurring basis | Asset-backed securities
Assets
Fixed maturities available-for-sale:	1,676,750	1,312,379
Recurring basis | Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:	70,919	140,992
Recurring basis | Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:	1,386,952	1,225,282
Fixed maturities held-for-trading:	1,081	1,080
Recurring basis | Collateralized debt obligations
Assets
Fixed maturities available-for-sale:	728,770	361,527
Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Short-term investments	320,398	267,851
Collateral under securities lending agreements	1,870
Collateral under derivative counterparty collateral agreements	41,524	103,214
Separate account assets	16,255,644	15,407,992
Total assets	16,619,436	15,779,057
Liabilities
Collateral under derivative counterparty collateral agreements		103,214
Separate account liabilities	18	55
Total liabilities	18	103,269
Recurring basis | Significant other observable inputs (Level 2)
Assets
Fixed maturities available-for-sale:	23,002,789	22,142,064
Fixed maturities held-for-trading:	60,328	514,738
Short-term investments	502,673	36,137
Collateral under securities lending agreements	72,925
Total derivative instruments	98,777	146,385
Separate account assets	11,213,991	11,199,924
Total assets	34,951,483	34,039,248
Liabilities
Total derivative instruments	82,940	38,281
Separate account liabilities	521,723	336,468
Total liabilities	604,663	374,749
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	29,482	33,390
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	30,874	53,641
Liabilities
Total derivative instruments	35,184	8,294
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	10,147	8,982
Liabilities
Total derivative instruments	12,624	13,340
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments	156	354
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments	925
Liabilities
Total derivative instruments	4,076
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	27,193	50,018
Liabilities
Total derivative instruments	31,056	16,647
Recurring basis | Significant other observable inputs (Level 2) | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale:	1,543,460	3,035,112
Fixed maturities held-for-trading:	20,291	458,067
Recurring basis | Significant other observable inputs (Level 2) | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:	2,110,818	2,098,662
Recurring basis | Significant other observable inputs (Level 2) | Corporate debt securities
Assets
Fixed maturities available-for-sale:	15,485,120	13,968,110
Fixed maturities held-for-trading:	38,956	55,591
Recurring basis | Significant other observable inputs (Level 2) | Asset-backed securities
Assets
Fixed maturities available-for-sale:	1,676,750	1,312,379
Recurring basis | Significant other observable inputs (Level 2) | Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:	70,919	140,992
Recurring basis | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:	1,386,952	1,225,282
Fixed maturities held-for-trading:	1,081	1,080
Recurring basis | Significant other observable inputs (Level 2) | Collateralized debt obligations
Assets
Fixed maturities available-for-sale:	728,770	361,527
Recurring basis | Significant unobservable inputs (Level 3)
Assets
Fixed maturities available-for-sale:	10,080	11,639
Total assets	10,080	11,639
Liabilities
Embedded derivatives - GLWB	10,221	5,712
Total liabilities	10,221	5,712
Recurring basis | Significant unobservable inputs (Level 3) | Corporate debt securities
Assets
Fixed maturities available-for-sale:	10,080	11,639
Recurring basis | Significant unobservable inputs (Level 3) | Asset-backed securities
Assets
Fixed maturities available-for-sale:	$ 0	$ 0