Fair Value Measurements - Recurring by FV hierarchy (Details) - USD ($)	Jun. 30, 2017	Dec. 31, 2016
Assets
Fixed maturities available-for-sale:	$ 22,242,518,000	$ 22,153,703,000
Fixed maturities held-for-trading:	182,554,000	514,738,000
Short-term investments	917,716,000	303,988,000
Collateral under securities lending agreements	77,614,000	0
Collateral under derivative counterparty collateral agreements	910,000	0
Total derivative instruments	119,183,000	146,385,000
Separate account assets	27,856,199,000	27,037,765,000
Liabilities
Collateral under derivative counterparty collateral agreements	60,919,000	103,214,000
Total derivative instruments	61,971,000	38,281,000
Separate account liabilities	27,856,199,000	27,037,765,000
Derivative instruments designated as hedges:
Assets
Total derivative instruments	70,253,000	87,031,000
Liabilities
Total derivative instruments	20,052,000	8,294,000
Derivative instruments not designated as hedges:
Assets
Total derivative instruments	48,930,000	59,354,000
Liabilities
Total derivative instruments	41,919,000	29,987,000
Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	10,296,000	8,982,000
Liabilities
Total derivative instruments	12,492,000	13,340,000
Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments	204,000	354,000
Liabilities
Total derivative instruments	0	0
Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments	2,843,000
Liabilities
Total derivative instruments	5,423,000
Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	35,587,000	50,018,000
Liabilities
Total derivative instruments	24,004,000	16,647,000
U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale:	1,431,635,000	3,035,112,000
Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:	2,108,567,000	2,098,662,000
Corporate debt securities
Assets
Fixed maturities available-for-sale:	15,026,395,000	13,979,749,000
Asset-backed securities
Assets
Fixed maturities available-for-sale:	1,722,178,000	1,312,379,000
Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:	75,090,000	140,992,000
Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:	1,297,687,000	1,225,282,000
Collateralized debt obligations
Assets
Fixed maturities available-for-sale:	580,966,000	361,527,000
Recurring basis
Assets
Fixed maturities available-for-sale:	22,242,518,000	22,153,703,000
Fixed maturities held-for-trading:	182,554,000	514,738,000
Short-term investments	917,716,000	303,988,000
Collateral under securities lending agreements	77,614,000
Collateral under derivative counterparty collateral agreements	61,829,000	103,214,000
Total derivative instruments	119,183,000	146,385,000
Separate account assets	27,856,199,000	27,037,765,000
Total assets	51,457,613,000	50,259,793,000
Separate account assets excluded	432,000,000	430,000,000
Liabilities
Collateral under derivative counterparty collateral agreements		103,214,000
Total derivative instruments	61,971,000	38,281,000
Embedded derivatives - GLWB	9,595,000	5,712,000
Separate account liabilities	444,835,000	336,523,000
Total liabilities	516,401,000	483,730,000
Recurring basis | Derivative instruments designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	29,929,000	33,390,000
Recurring basis | Derivative instruments designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	40,324,000	53,641,000
Liabilities
Total derivative instruments	20,052,000	8,294,000
Recurring basis | Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	10,296,000	8,982,000
Liabilities
Total derivative instruments	12,492,000	13,340,000
Recurring basis | Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments	204,000	354,000
Recurring basis | Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments	2,843,000
Liabilities
Total derivative instruments	5,423,000
Recurring basis | Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	35,587,000	50,018,000
Liabilities
Total derivative instruments	24,004,000	16,647,000
Recurring basis | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale:	1,431,635,000	3,035,112,000
Fixed maturities held-for-trading:	126,590,000	458,067,000
Recurring basis | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:	2,108,567,000	2,098,662,000
Recurring basis | Corporate debt securities
Assets
Fixed maturities available-for-sale:	15,026,395,000	13,979,749,000
Fixed maturities held-for-trading:	54,889,000	55,591,000
Recurring basis | Asset-backed securities
Assets
Fixed maturities available-for-sale:	1,722,178,000	1,312,379,000
Recurring basis | Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:	75,090,000	140,992,000
Recurring basis | Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:	1,297,687,000	1,225,282,000
Fixed maturities held-for-trading:	1,075,000	1,080,000
Recurring basis | Collateralized debt obligations
Assets
Fixed maturities available-for-sale:	580,966,000	361,527,000
Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Short-term investments	406,459,000	267,851,000
Collateral under securities lending agreements	0
Collateral under derivative counterparty collateral agreements	61,829,000	103,214,000
Separate account assets	16,057,719,000	15,407,992,000
Total assets	16,526,007,000	15,779,057,000
Liabilities
Collateral under derivative counterparty collateral agreements		103,214,000
Separate account liabilities	4,000	55,000
Total liabilities	4,000	103,269,000
Recurring basis | Significant other observable inputs (Level 2)
Assets
Fixed maturities available-for-sale:	22,231,815,000	22,142,064,000
Fixed maturities held-for-trading:	182,554,000	514,738,000
Short-term investments	511,257,000	36,137,000
Collateral under securities lending agreements	77,614,000
Total derivative instruments	119,183,000	146,385,000
Separate account assets	11,366,695,000	11,199,924,000
Total assets	34,489,118,000	34,039,248,000
Liabilities
Total derivative instruments	61,971,000	38,281,000
Separate account liabilities	444,831,000	336,468,000
Total liabilities	506,802,000	374,749,000
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	29,929,000	33,390,000
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	40,324,000	53,641,000
Liabilities
Total derivative instruments	20,052,000	8,294,000
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	10,296,000	8,982,000
Liabilities
Total derivative instruments	12,492,000	13,340,000
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments	204,000	354,000
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments	2,843,000
Liabilities
Total derivative instruments	5,423,000
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	35,587,000	50,018,000
Liabilities
Total derivative instruments	24,004,000	16,647,000
Recurring basis | Significant other observable inputs (Level 2) | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale:	1,431,635,000	3,035,112,000
Fixed maturities held-for-trading:	126,590,000	458,067,000
Recurring basis | Significant other observable inputs (Level 2) | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:	2,108,567,000	2,098,662,000
Recurring basis | Significant other observable inputs (Level 2) | Corporate debt securities
Assets
Fixed maturities available-for-sale:	15,015,692,000	13,968,110,000
Fixed maturities held-for-trading:	54,889,000	55,591,000
Recurring basis | Significant other observable inputs (Level 2) | Asset-backed securities
Assets
Fixed maturities available-for-sale:	1,722,178,000	1,312,379,000
Recurring basis | Significant other observable inputs (Level 2) | Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:	75,090,000	140,992,000
Recurring basis | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:	1,297,687,000	1,225,282,000
Fixed maturities held-for-trading:	1,075,000	1,080,000
Recurring basis | Significant other observable inputs (Level 2) | Collateralized debt obligations
Assets
Fixed maturities available-for-sale:	580,966,000	361,527,000
Recurring basis | Significant unobservable inputs (Level 3)
Assets
Fixed maturities available-for-sale:	10,703,000	11,639,000
Total assets	10,703,000	11,639,000
Liabilities
Embedded derivatives - GLWB	9,595,000	5,712,000
Total liabilities	9,595,000	5,712,000
Recurring basis | Significant unobservable inputs (Level 3) | Corporate debt securities
Assets
Fixed maturities available-for-sale:	10,703,000	11,639,000
Recurring basis | Significant unobservable inputs (Level 3) | Asset-backed securities
Assets
Fixed maturities available-for-sale:	$ 0	$ 0