Fair Value Measurements - Recurring by FV hierarchy (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets
Fixed maturities available-for-sale:	$ 22,762,737	$ 23,593,139
Fixed maturities held-for-trading:	19,519	21,059
Short-term investments	294,314	350,266
Limited partnership interests	63,586	45,540
Collateral under securities lending agreements	80,013	0
Collateral under derivative counterparty collateral agreements	16,310	42,750
Total derivative instruments	62,869	51,359
Separate account assets	26,676,615	27,660,571
Liabilities
Payable under securities lending agreements	80,013	0
Collateral under derivative counterparty collateral agreements	10,612	4,911
Total derivative instruments	73,550	94,543
Separate account liabilities	26,676,615	27,660,571
Derivative instruments designated as hedges:
Assets
Total derivative instruments	36,388	27,434
Liabilities
Total derivative instruments	39,325	51,316
Derivative instruments not designated as hedges:
Assets
Total derivative instruments	26,481	23,925
Liabilities
Total derivative instruments	34,225	43,227
Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	1,461	3,530
Liabilities
Total derivative instruments	884	1,628
Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	20,292	20,320
Liabilities
Total derivative instruments	32,070	41,599
Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments	163	75
Liabilities
Total derivative instruments	0	0
Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments	4,565
Liabilities
Total derivative instruments	1,271
U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale:	1,356,173	1,871,642
Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:	2,003,422	2,090,972
Corporate debt securities
Assets
Fixed maturities available-for-sale:	15,438,738	15,706,087
Liabilities
Payable under securities lending agreements	80,013
Asset-backed securities
Assets
Fixed maturities available-for-sale:	1,653,801	1,717,976
Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:	57,825	64,984
Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:	1,319,800	1,357,609
Collateralized debt obligations
Assets
Fixed maturities available-for-sale:	932,978	783,869
Recurring basis
Assets
Fixed maturities available-for-sale:	22,762,737	23,593,139
Fixed maturities held-for-trading:	19,519	21,059
Short-term investments	294,314	350,266
Limited partnership interests	63,586
Equity investments	42,575
Collateral under securities lending agreements	80,013
Collateral under derivative counterparty collateral agreements	39,865	57,820
Total derivative instruments	62,869	51,359
Separate account assets	26,676,615	27,660,571
Total assets	49,935,932	51,734,214
Separate account assets excluded	450,000	400,000
Liabilities
Payable under securities lending agreements	80,013
Collateral under derivative counterparty collateral agreements	12,696	5,490
Total derivative instruments	73,550	94,543
Embedded derivatives - GLWB	513	11,095
Separate account liabilities	431,960	409,274
Total liabilities	598,732	520,402
Recurring basis | Derivative instruments designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	5,917	7,476
Recurring basis | Derivative instruments designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	30,471	19,958
Liabilities
Total derivative instruments	39,325	51,316
Recurring basis | Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	1,461	3,530
Liabilities
Total derivative instruments	884	1,628
Recurring basis | Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	20,292	20,320
Liabilities
Total derivative instruments	32,070	41,599
Recurring basis | Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments	163	75
Recurring basis | Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments	4,565
Liabilities
Total derivative instruments	1,271
Recurring basis | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale:	1,356,173	1,871,642
Fixed maturities held-for-trading:	15,481	16,836
Recurring basis | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:	2,003,422	2,090,972
Recurring basis | Corporate debt securities
Assets
Fixed maturities available-for-sale:	15,438,738	15,706,087
Fixed maturities held-for-trading:	3,001	3,156
Recurring basis | Asset-backed securities
Assets
Fixed maturities available-for-sale:	1,653,801	1,717,976
Recurring basis | Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:	57,825	64,984
Recurring basis | Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:	1,319,800	1,357,609
Fixed maturities held-for-trading:	1,037	1,067
Recurring basis | Collateralized debt obligations
Assets
Fixed maturities available-for-sale:	932,978	783,869
Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Fixed maturities available-for-sale:	0	0
Fixed maturities held-for-trading:	0	0
Short-term investments	260,967	288,302
Limited partnership interests	0
Equity investments	37,564
Collateral under securities lending agreements	0
Collateral under derivative counterparty collateral agreements	39,865	57,820
Total derivative instruments	0	0
Separate account assets	16,089,089	16,523,630
Total assets	16,389,921	16,869,752
Liabilities
Payable under securities lending agreements	0
Collateral under derivative counterparty collateral agreements	12,696	5,490
Total derivative instruments	0	0
Embedded derivatives - GLWB	0	0
Separate account liabilities	0	8
Total liabilities	12,696	5,498
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Derivative instruments designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Derivative instruments designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	0	0
Liabilities
Total derivative instruments	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	0	0
Liabilities
Total derivative instruments	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	0	0
Liabilities
Total derivative instruments	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments	0
Liabilities
Total derivative instruments	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale:	0	0
Fixed maturities held-for-trading:	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Corporate debt securities
Assets
Fixed maturities available-for-sale:	0	0
Fixed maturities held-for-trading:	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Asset-backed securities
Assets
Fixed maturities available-for-sale:	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:	0	0
Fixed maturities held-for-trading:	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Collateralized debt obligations
Assets
Fixed maturities available-for-sale:	0	0
Recurring basis | Significant other observable inputs (Level 2)
Assets
Fixed maturities available-for-sale:	22,753,767	23,583,401
Fixed maturities held-for-trading:	19,519	21,059
Short-term investments	33,347	61,964
Limited partnership interests	0
Equity investments	5,011
Collateral under securities lending agreements	80,013
Collateral under derivative counterparty collateral agreements	0	0
Total derivative instruments	62,869	51,359
Separate account assets	10,137,807	10,736,532
Total assets	33,087,322	34,454,315
Liabilities
Payable under securities lending agreements	80,013
Collateral under derivative counterparty collateral agreements	0	0
Total derivative instruments	73,550	94,543
Embedded derivatives - GLWB	0	0
Separate account liabilities	431,960	409,266
Total liabilities	585,523	503,809
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	5,917	7,476
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	30,471	19,958
Liabilities
Total derivative instruments	39,325	51,316
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	1,461	3,530
Liabilities
Total derivative instruments	884	1,628
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	20,292	20,320
Liabilities
Total derivative instruments	32,070	41,599
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments	163	75
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments	4,565
Liabilities
Total derivative instruments	1,271
Recurring basis | Significant other observable inputs (Level 2) | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale:	1,356,173	1,871,642
Fixed maturities held-for-trading:	15,481	16,836
Recurring basis | Significant other observable inputs (Level 2) | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:	2,003,422	2,090,972
Recurring basis | Significant other observable inputs (Level 2) | Corporate debt securities
Assets
Fixed maturities available-for-sale:	15,429,768	15,696,349
Fixed maturities held-for-trading:	3,001	3,156
Recurring basis | Significant other observable inputs (Level 2) | Asset-backed securities
Assets
Fixed maturities available-for-sale:	1,653,801	1,717,976
Recurring basis | Significant other observable inputs (Level 2) | Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:	57,825	64,984
Recurring basis | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:	1,319,800	1,357,609
Fixed maturities held-for-trading:	1,037	1,067
Recurring basis | Significant other observable inputs (Level 2) | Collateralized debt obligations
Assets
Fixed maturities available-for-sale:	932,978	783,869
Recurring basis | Significant unobservable inputs (Level 3)
Assets
Fixed maturities available-for-sale:	8,970	9,738
Fixed maturities held-for-trading:	0	0
Short-term investments	0	0
Limited partnership interests	0
Equity investments	0
Collateral under securities lending agreements	0
Collateral under derivative counterparty collateral agreements	0	0
Total derivative instruments	0	0
Separate account assets	0	0
Total assets	8,970	9,738
Liabilities
Payable under securities lending agreements	0
Collateral under derivative counterparty collateral agreements	0	0
Total derivative instruments	0	0
Embedded derivatives - GLWB	513	11,095
Separate account liabilities	0	0
Total liabilities	513	11,095
Recurring basis | Significant unobservable inputs (Level 3) | Derivative instruments designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	0	0
Recurring basis | Significant unobservable inputs (Level 3) | Derivative instruments designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	0	0
Liabilities
Total derivative instruments	0	0
Recurring basis | Significant unobservable inputs (Level 3) | Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	0	0
Liabilities
Total derivative instruments	0	0
Recurring basis | Significant unobservable inputs (Level 3) | Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	0	0
Liabilities
Total derivative instruments	0	0
Recurring basis | Significant unobservable inputs (Level 3) | Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments	0	0
Recurring basis | Significant unobservable inputs (Level 3) | Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments	0
Liabilities
Total derivative instruments	0
Recurring basis | Significant unobservable inputs (Level 3) | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale:	0	0
Fixed maturities held-for-trading:	0	0
Recurring basis | Significant unobservable inputs (Level 3) | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:	0	0
Recurring basis | Significant unobservable inputs (Level 3) | Corporate debt securities
Assets
Fixed maturities available-for-sale:	8,970	9,738
Fixed maturities held-for-trading:	0	0
Recurring basis | Significant unobservable inputs (Level 3) | Asset-backed securities
Assets
Fixed maturities available-for-sale:	0	0
Recurring basis | Significant unobservable inputs (Level 3) | Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:	0	0
Recurring basis | Significant unobservable inputs (Level 3) | Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:	0	0
Fixed maturities held-for-trading:	0	0
Recurring basis | Significant unobservable inputs (Level 3) | Collateralized debt obligations
Assets
Fixed maturities available-for-sale:	$ 0	$ 0