Fair Value Measurements - Recurring by FV hierarchy (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets
Fixed maturities available-for-sale:	$ 22,550,302	$ 23,593,139
Fixed maturities held-for-trading:	141,943	21,059
Short-term investments	415,189	350,266
Limited partnership interests	71,895	45,540
Collateral under securities lending agreements	66,784	0
Collateral under derivative counterparty collateral agreements	20,588	42,750
Total derivative instruments	60,318	51,359
Separate account assets	26,142,156	27,660,571
Liabilities
Payable under securities lending agreements	66,784	0
Collateral under derivative counterparty collateral agreements	8,939	4,911
Total derivative instruments	73,384	94,543
Separate account liabilities	26,142,156	27,660,571
Derivative instruments designated as hedges:
Assets
Total derivative instruments	36,062	27,434
Liabilities
Total derivative instruments	37,542	51,316
Derivative instruments not designated as hedges:
Assets
Total derivative instruments	24,256	23,925
Liabilities
Total derivative instruments	35,842	43,227
Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	681	3,530
Liabilities
Total derivative instruments	943	1,628
Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	22,435	20,320
Liabilities
Total derivative instruments	28,839	41,599
Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments	164	75
Liabilities
Total derivative instruments	0	0
Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments	976
Liabilities
Total derivative instruments	6,060
U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale:	1,426,095	1,871,642
Liabilities
Payable under securities lending agreements	7,700	0
Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:	1,959,125	2,090,972
Corporate debt securities
Assets
Fixed maturities available-for-sale:	15,339,908	15,706,087
Liabilities
Payable under securities lending agreements	59,084	0
Asset-backed securities
Assets
Fixed maturities available-for-sale:	1,568,739	1,717,976
Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:	75,247	64,984
Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:	1,245,498	1,357,609
Collateralized debt obligations
Assets
Fixed maturities available-for-sale:	935,690	783,869
Recurring basis
Assets
Fixed maturities available-for-sale:	22,550,302	23,593,139
Fixed maturities held-for-trading:	141,943	21,059
Short-term investments	415,189	350,266
Limited partnership interests	71,895
Equity investments	43,774
Collateral under securities lending agreements	66,784
Collateral under derivative counterparty collateral agreements	51,090	57,820
Total derivative instruments	60,318	51,359
Embedded derivatives - GLWB	6,849
Separate account assets	26,142,156	27,660,571
Total assets	49,550,300	51,734,214
Liabilities
Payable under securities lending agreements	66,784
Collateral under derivative counterparty collateral agreements	11,960	5,490
Total derivative instruments	73,384	94,543
Embedded derivatives - GLWB		11,095
Separate account liabilities	217,062	409,274
Total liabilities	369,190	520,402
Separate account assets excluded	445,000	400,000
Recurring basis | Derivative instruments designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	5,037	7,476
Recurring basis | Derivative instruments designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	31,025	19,958
Liabilities
Total derivative instruments	37,542	51,316
Recurring basis | Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	681	3,530
Liabilities
Total derivative instruments	943	1,628
Recurring basis | Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	22,435	20,320
Liabilities
Total derivative instruments	28,839	41,599
Recurring basis | Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments	164	75
Recurring basis | Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments	976
Liabilities
Total derivative instruments	6,060
Recurring basis | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale:	1,426,095	1,871,642
Fixed maturities held-for-trading:	137,866	16,836
Recurring basis | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:	1,959,125	2,090,972
Recurring basis | Corporate debt securities
Assets
Fixed maturities available-for-sale:	15,339,908	15,706,087
Fixed maturities held-for-trading:	3,044	3,156
Recurring basis | Asset-backed securities
Assets
Fixed maturities available-for-sale:	1,568,739	1,717,976
Recurring basis | Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:	75,247	64,984
Recurring basis | Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:	1,245,498	1,357,609
Fixed maturities held-for-trading:	1,033	1,067
Recurring basis | Collateralized debt obligations
Assets
Fixed maturities available-for-sale:	935,690	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	369,539	288,302
Limited partnership interests	0
Equity investments	38,374
Collateral under securities lending agreements	0
Collateral under derivative counterparty collateral agreements	51,090	57,820
Total derivative instruments	0	0
Embedded derivatives - GLWB	0
Separate account assets	15,781,604	16,523,630
Total assets	16,240,607	16,869,752
Liabilities
Payable under securities lending agreements	0
Collateral under derivative counterparty collateral agreements	11,960	5,490
Total derivative instruments	0	0
Embedded derivatives - GLWB		0
Separate account liabilities	0	8
Total liabilities	11,960	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:	22,541,679	23,583,401
Fixed maturities held-for-trading:	141,943	21,059
Short-term investments	45,650	61,964
Limited partnership interests	0
Equity investments	5,400
Collateral under securities lending agreements	66,784
Collateral under derivative counterparty collateral agreements	0	0
Total derivative instruments	60,318	51,359
Embedded derivatives - GLWB	0
Separate account assets	9,915,781	10,736,532
Total assets	32,777,555	34,454,315
Liabilities
Payable under securities lending agreements	66,784
Collateral under derivative counterparty collateral agreements	0	0
Total derivative instruments	73,384	94,543
Embedded derivatives - GLWB		0
Separate account liabilities	217,062	409,266
Total liabilities	357,230	503,809
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	5,037	7,476
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	31,025	19,958
Liabilities
Total derivative instruments	37,542	51,316
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	681	3,530
Liabilities
Total derivative instruments	943	1,628
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	22,435	20,320
Liabilities
Total derivative instruments	28,839	41,599
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments	164	75
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments	976
Liabilities
Total derivative instruments	6,060
Recurring basis | Significant other observable inputs (Level 2) | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale:	1,426,095	1,871,642
Fixed maturities held-for-trading:	137,866	16,836
Recurring basis | Significant other observable inputs (Level 2) | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:	1,959,125	2,090,972
Recurring basis | Significant other observable inputs (Level 2) | Corporate debt securities
Assets
Fixed maturities available-for-sale:	15,331,285	15,696,349
Fixed maturities held-for-trading:	3,044	3,156
Recurring basis | Significant other observable inputs (Level 2) | Asset-backed securities
Assets
Fixed maturities available-for-sale:	1,568,739	1,717,976
Recurring basis | Significant other observable inputs (Level 2) | Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:	75,247	64,984
Recurring basis | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:	1,245,498	1,357,609
Fixed maturities held-for-trading:	1,033	1,067
Recurring basis | Significant other observable inputs (Level 2) | Collateralized debt obligations
Assets
Fixed maturities available-for-sale:	935,690	783,869
Recurring basis | Significant unobservable inputs (Level 3)
Assets
Fixed maturities available-for-sale:	8,623	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
Embedded derivatives - GLWB	6,849
Separate account assets	0	0
Total assets	15,472	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		11,095
Separate account liabilities	0	0
Total liabilities	0	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:	8,623	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