Fair Value Measurements - Recurring by FV hierarchy (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Assets
Fixed maturities available-for-sale:	$ 20,826,141	$ 20,531,627
Fixed maturities held-for-trading:	398,880	615,839
Short-term investments	2,314,307	267,026
Collateral under securities lending agreements	99,404	0
Collateral under derivative counterparty collateral agreements	28,526	50,924
Total derivative instruments	147,366	68,600
Separate account assets	27,775,827	26,631,193
Liabilities
Payable under securities lending agreements	99,404	0
Collateral under derivative counterparty collateral agreements	80,630	19,060
Total derivative instruments	69,944	76,373
Separate account liabilities	27,775,827	26,631,193
Derivative instruments designated as hedges:
Assets
Total derivative instruments	64,774	40,579
Liabilities
Total derivative instruments	32,432	22
Derivative instruments not designated as hedges:
Assets
Total derivative instruments	82,592	28,021
Liabilities
Total derivative instruments	37,512	76,351
Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	32,982	8,295
Liabilities
Total derivative instruments	10,061	5,055
Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments	191	189
Liabilities
Total derivative instruments	0	0
Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments	7,891
Liabilities
Total derivative instruments	623
Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	41,528	19,537
Liabilities
Total derivative instruments	26,828	71,296
U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale:	1,128,002	3,341,752
Liabilities
Payable under securities lending agreements	6,220	0
Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:	2,251,210	2,219,173
Foreign government securities
Assets
Fixed maturities available-for-sale:	2,164	2,286
Liabilities
Payable under securities lending agreements	1,063	0
Corporate debt securities
Assets
Fixed maturities available-for-sale:	14,404,087	12,505,712
Liabilities
Payable under securities lending agreements	92,121	0
Asset-backed securities
Assets
Fixed maturities available-for-sale:	1,596,662	1,311,370
Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:	96,717	125,372
Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:	1,217,078	1,016,908
Collateralized debt obligations
Assets
Fixed maturities available-for-sale:	130,221	9,054
Recurring basis
Assets
Fixed maturities available-for-sale:	20,826,141	20,531,627
Fixed maturities held-for-trading:	398,880	615,839
Short-term investments	2,314,307	267,026
Collateral under securities lending agreements	99,404
Collateral under derivative counterparty collateral agreements	109,156	69,984
Total derivative instruments	147,366	68,600
Separate account assets	27,775,827	26,631,193
Total assets	51,671,081	48,184,269
Separate account assets excluded	457,000	534,000
Liabilities
Payable under securities lending agreements	99,404
Collateral under derivative counterparty collateral agreements	80,630	19,060
Total derivative instruments	69,944	76,373
Embedded derivatives - GLWB	31,277	11,257
Separate account liabilities	517,574	290,317
Total liabilities	798,829	397,007
Recurring basis | Derivative instruments designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	13,490	11,843
Liabilities
Total derivative instruments	27,342
Recurring basis | Derivative instruments designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	51,284	28,736
Liabilities
Total derivative instruments	5,090	22
Recurring basis | Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	32,982	8,295
Liabilities
Total derivative instruments	10,061	5,055
Recurring basis | Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments	191	189
Recurring basis | Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments	7,891
Liabilities
Total derivative instruments	623
Recurring basis | Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	41,528	19,537
Liabilities
Total derivative instruments	26,828	71,296
Recurring basis | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale:	1,128,002	3,341,752
Fixed maturities held-for-trading:	341,368	558,208
Recurring basis | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:	2,251,210	2,219,173
Recurring basis | Foreign government securities
Assets
Fixed maturities available-for-sale:	2,164	2,286
Recurring basis | Corporate debt securities
Assets
Fixed maturities available-for-sale:	14,404,087	12,505,712
Fixed maturities held-for-trading:	56,390	56,566
Recurring basis | Asset-backed securities
Assets
Fixed maturities available-for-sale:	1,596,662	1,311,370
Recurring basis | Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:	96,717	125,372
Recurring basis | Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:	1,217,078	1,016,908
Fixed maturities held-for-trading:	1,122	1,065
Recurring basis | Collateralized debt obligations
Assets
Fixed maturities available-for-sale:	130,221	9,054
Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Short-term investments	338,051	132,288
Collateral under securities lending agreements	99,404
Collateral under derivative counterparty collateral agreements	109,156	69,984
Separate account assets	15,580,267	15,249,966
Total assets	16,126,878	15,452,238
Liabilities
Payable under securities lending agreements	99,404
Collateral under derivative counterparty collateral agreements	80,630	19,060
Separate account liabilities	4	24
Total liabilities	180,038	19,084
Recurring basis | Significant other observable inputs (Level 2)
Assets
Fixed maturities available-for-sale:	20,811,434	20,527,089
Fixed maturities held-for-trading:	398,880	615,839
Short-term investments	1,976,256	134,738
Total derivative instruments	147,366	68,600
Separate account assets	11,738,559	10,847,266
Total assets	35,072,495	32,193,532
Liabilities
Total derivative instruments	69,944	76,373
Separate account liabilities	517,570	290,293
Total liabilities	587,514	366,666
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	13,490	11,843
Liabilities
Total derivative instruments	27,342
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	51,284	28,736
Liabilities
Total derivative instruments	5,090	22
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	32,982	8,295
Liabilities
Total derivative instruments	10,061	5,055
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments	191	189
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments	7,891
Liabilities
Total derivative instruments	623
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	41,528	19,537
Liabilities
Total derivative instruments	26,828	71,296
Recurring basis | Significant other observable inputs (Level 2) | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale:	1,128,002	3,341,752
Fixed maturities held-for-trading:	341,368	558,208
Recurring basis | Significant other observable inputs (Level 2) | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:	2,251,210	2,219,173
Recurring basis | Significant other observable inputs (Level 2) | Foreign government securities
Assets
Fixed maturities available-for-sale:	2,164	2,286
Recurring basis | Significant other observable inputs (Level 2) | Corporate debt securities
Assets
Fixed maturities available-for-sale:	14,389,380	12,501,174
Fixed maturities held-for-trading:	56,390	56,566
Recurring basis | Significant other observable inputs (Level 2) | Asset-backed securities
Assets
Fixed maturities available-for-sale:	1,596,662	1,311,370
Recurring basis | Significant other observable inputs (Level 2) | Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:	96,717	125,372
Recurring basis | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:	1,217,078	1,016,908
Fixed maturities held-for-trading:	1,122	1,065
Recurring basis | Significant other observable inputs (Level 2) | Collateralized debt obligations
Assets
Fixed maturities available-for-sale:	130,221	9,054
Recurring basis | Significant unobservable inputs (Level 3)
Assets
Fixed maturities available-for-sale:	14,707	4,538
Total assets	14,707	4,538
Liabilities
Embedded derivatives - GLWB	31,277	11,257
Total liabilities	31,277	11,257
Recurring basis | Significant unobservable inputs (Level 3) | Corporate debt securities
Assets
Fixed maturities available-for-sale:	14,707	4,538
Recurring basis | Significant unobservable inputs (Level 3) | Asset-backed securities
Assets
Fixed maturities available-for-sale:	$ 0	$ 0