Fair Value Measurements - Recurring by FV hierarchy (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Assets
Fixed maturities available-for-sale:	$ 22,042,336	$ 22,153,703
Fixed maturities held-for-trading:	185,617	514,738
Short-term investments	744,063	303,988
Collateral under securities lending agreements	94,531	0
Collateral under derivative counterparty collateral agreements	0	0
Total derivative instruments	139,300	146,385
Separate account assets	27,597,906	27,037,765
Liabilities
Payable under securities lending agreements	94,531	0
Collateral under derivative counterparty collateral agreements	80,145	103,214
Total derivative instruments	49,768	38,281
Separate account liabilities	27,597,906	27,037,765
Derivative instruments designated as hedges:
Assets
Total derivative instruments	80,960	87,031
Liabilities
Total derivative instruments	10,921	8,294
Derivative instruments not designated as hedges:
Assets
Total derivative instruments	58,340	59,354
Liabilities
Total derivative instruments	38,847	29,987
Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	8,187	8,982
Liabilities
Total derivative instruments	14,895	13,340
Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments	265	354
Liabilities
Total derivative instruments	0	0
Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments	7,817
Liabilities
Total derivative instruments	1,033
Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	42,071	50,018
Liabilities
Total derivative instruments	22,919	16,647
U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale:	1,840,368	3,035,112
Liabilities
Payable under securities lending agreements	1,025	0
Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:	2,100,099	2,098,662
Corporate debt securities
Assets
Fixed maturities available-for-sale:	14,553,577	13,979,749
Liabilities
Payable under securities lending agreements	93,506	0
Asset-backed securities
Assets
Fixed maturities available-for-sale:	1,528,264	1,312,379
Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:	127,726	140,992
Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:	1,358,065	1,225,282
Collateralized debt obligations
Assets
Fixed maturities available-for-sale:	534,237	361,527
Recurring basis
Assets
Fixed maturities available-for-sale:	22,042,336	22,153,703
Fixed maturities held-for-trading:	185,617	514,738
Short-term investments	744,063	303,988
Collateral under securities lending agreements	94,531
Collateral under derivative counterparty collateral agreements	80,145	103,214
Total derivative instruments	139,300	146,385
Separate account assets	27,597,906	27,037,765
Total assets	50,883,898	50,259,793
Separate account assets excluded	418,000	430,000
Liabilities
Payable under securities lending agreements	94,531
Collateral under derivative counterparty collateral agreements	80,145	103,214
Total derivative instruments	49,768	38,281
Embedded derivatives - GLWB	4,042	5,712
Separate account liabilities	409,070	336,523
Total liabilities	637,556	483,730
Recurring basis | Derivative instruments designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	37,308	33,390
Recurring basis | Derivative instruments designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	43,652	53,641
Liabilities
Total derivative instruments	10,921	8,294
Recurring basis | Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	8,187	8,982
Liabilities
Total derivative instruments	14,895	13,340
Recurring basis | Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments	265	354
Recurring basis | Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments	7,817
Liabilities
Total derivative instruments	1,033
Recurring basis | Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	42,071	50,018
Liabilities
Total derivative instruments	22,919	16,647
Recurring basis | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale:	1,840,368	3,035,112
Fixed maturities held-for-trading:	129,296	458,067
Recurring basis | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:	2,100,099	2,098,662
Recurring basis | Corporate debt securities
Assets
Fixed maturities available-for-sale:	14,553,577	13,979,749
Fixed maturities held-for-trading:	55,249	55,591
Recurring basis | Asset-backed securities
Assets
Fixed maturities available-for-sale:	1,528,264	1,312,379
Recurring basis | Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:	127,726	140,992
Recurring basis | Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:	1,358,065	1,225,282
Fixed maturities held-for-trading:	1,072	1,080
Recurring basis | Collateralized debt obligations
Assets
Fixed maturities available-for-sale:	534,237	361,527
Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Short-term investments	262,866	267,851
Collateral under securities lending agreements	94,531
Collateral under derivative counterparty collateral agreements	80,145	103,214
Separate account assets	15,849,064	15,407,992
Total assets	16,286,606	15,779,057
Liabilities
Payable under securities lending agreements	94,531
Collateral under derivative counterparty collateral agreements	80,145	103,214
Separate account liabilities	26	55
Total liabilities	174,702	103,269
Recurring basis | Significant other observable inputs (Level 2)
Assets
Fixed maturities available-for-sale:	22,031,405	22,142,064
Fixed maturities held-for-trading:	185,617	514,738
Short-term investments	481,197	36,137
Total derivative instruments	139,300	146,385
Separate account assets	11,330,972	11,199,924
Total assets	34,168,491	34,039,248
Liabilities
Total derivative instruments	49,768	38,281
Separate account liabilities	409,044	336,468
Total liabilities	458,812	374,749
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	37,308	33,390
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	43,652	53,641
Liabilities
Total derivative instruments	10,921	8,294
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	8,187	8,982
Liabilities
Total derivative instruments	14,895	13,340
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments	265	354
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments	7,817
Liabilities
Total derivative instruments	1,033
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	42,071	50,018
Liabilities
Total derivative instruments	22,919	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:	1,840,368	3,035,112
Fixed maturities held-for-trading:	129,296	458,067
Recurring basis | Significant other observable inputs (Level 2) | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:	2,100,099	2,098,662
Recurring basis | Significant other observable inputs (Level 2) | Corporate debt securities
Assets
Fixed maturities available-for-sale:	14,542,646	13,968,110
Fixed maturities held-for-trading:	55,249	55,591
Recurring basis | Significant other observable inputs (Level 2) | Asset-backed securities
Assets
Fixed maturities available-for-sale:	1,528,264	1,312,379
Recurring basis | Significant other observable inputs (Level 2) | Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:	127,726	140,992
Recurring basis | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:	1,358,065	1,225,282
Fixed maturities held-for-trading:	1,072	1,080
Recurring basis | Significant other observable inputs (Level 2) | Collateralized debt obligations
Assets
Fixed maturities available-for-sale:	534,237	361,527
Recurring basis | Significant unobservable inputs (Level 3)
Assets
Fixed maturities available-for-sale:	10,931	11,639
Total assets	10,931	11,639
Liabilities
Embedded derivatives - GLWB	4,042	5,712
Total liabilities	4,042	5,712
Recurring basis | Significant unobservable inputs (Level 3) | Corporate debt securities
Assets
Fixed maturities available-for-sale:	10,931	11,639
Recurring basis | Significant unobservable inputs (Level 3) | Asset-backed securities
Assets
Fixed maturities available-for-sale:	$ 0	$ 0