Fair Value Measurements - Recurring by FV hierarchy (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Assets
Fixed maturities available-for-sale:	$ 20,170,588	$ 20,531,627
Fixed maturities held-for-trading:	326,752	615,839
Short-term investments	2,413,192	267,026
Collateral under securities lending agreements	79,780	0
Collateral under derivative counterparty collateral agreements	25,590	50,924
Total derivative instruments	160,914	68,600
Separate account assets	27,251,404	26,631,193
Liabilities
Payable under securities lending agreements	79,780	0
Collateral under derivative counterparty collateral agreements	82,626	19,060
Total derivative instruments	70,011	76,373
Separate account liabilities	27,251,404	26,631,193
Derivative instruments designated as hedges:
Assets
Total derivative instruments	66,936	40,579
Liabilities
Total derivative instruments	25,582	22
Derivative instruments not designated as hedges:
Assets
Total derivative instruments	93,978	28,021
Liabilities
Total derivative instruments	44,429	76,351
Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	33,083	8,295
Liabilities
Total derivative instruments	10,563	5,055
Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments	138	189
Liabilities
Total derivative instruments	0	0
Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments	20,582
Liabilities
Total derivative instruments	5,027
Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	40,175	19,537
Liabilities
Total derivative instruments	28,839	71,296
U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale:	1,181,290	3,341,752
Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:	2,273,641	2,219,173
Foreign government securities
Assets
Fixed maturities available-for-sale:	2,205	2,286
Corporate debt securities
Assets
Fixed maturities available-for-sale:	13,807,717	12,505,712
Asset-backed securities
Assets
Fixed maturities available-for-sale:	1,680,061	1,311,370
Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:	96,158	125,372
Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:	1,090,803	1,016,908
Collateralized debt obligations
Assets
Fixed maturities available-for-sale:	38,713	9,054
Recurring basis
Assets
Fixed maturities available-for-sale:	20,170,588	20,531,627
Fixed maturities held-for-trading:	326,752	615,839
Short-term investments	2,413,192	267,026
Collateral under securities lending agreements	79,780
Collateral under derivative counterparty collateral agreements	108,216	69,984
Total derivative instruments	160,914	68,600
Separate account assets	27,251,404	26,631,193
Total assets	50,510,846	48,184,269
Liabilities
Payable under securities lending agreements	79,780
Collateral under derivative counterparty collateral agreements	82,626	19,060
Total derivative instruments	70,011	76,373
Embedded derivatives - GLWB	30,687	11,257
Separate account liabilities	368,288	290,317
Total liabilities	631,392	397,007
Recurring basis | Derivative instruments designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	14,649	11,843
Liabilities
Total derivative instruments	24,150
Recurring basis | Derivative instruments designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	52,287	28,736
Liabilities
Total derivative instruments	1,432	22
Recurring basis | Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	33,083	8,295
Liabilities
Total derivative instruments	10,563	5,055
Recurring basis | Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments	138	189
Recurring basis | Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments	20,582
Liabilities
Total derivative instruments	5,027
Recurring basis | Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	40,175	19,537
Liabilities
Total derivative instruments	28,839	71,296
Recurring basis | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale:	1,181,290	3,341,752
Fixed maturities held-for-trading:	268,691	558,208
Recurring basis | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:	2,273,641	2,219,173
Recurring basis | Foreign government securities
Assets
Fixed maturities available-for-sale:	2,205	2,286
Recurring basis | Corporate debt securities
Assets
Fixed maturities available-for-sale:	13,807,717	12,505,712
Fixed maturities held-for-trading:	56,932	56,566
Recurring basis | Asset-backed securities
Assets
Fixed maturities available-for-sale:	1,680,061	1,311,370
Recurring basis | Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:	96,158	125,372
Recurring basis | Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:	1,090,803	1,016,908
Fixed maturities held-for-trading:	1,129	1,065
Recurring basis | Collateralized debt obligations
Assets
Fixed maturities available-for-sale:	38,713	9,054
Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Short-term investments	345,406	132,288
Collateral under securities lending agreements	79,780
Collateral under derivative counterparty collateral agreements	108,216	69,984
Separate account assets	15,193,369	15,249,966
Total assets	15,726,771	15,452,238
Liabilities
Payable under securities lending agreements	79,780
Collateral under derivative counterparty collateral agreements	82,626	19,060
Separate account liabilities	6	24
Total liabilities	162,412	19,084
Recurring basis | Significant other observable inputs (Level 2)
Assets
Fixed maturities available-for-sale:	20,155,532	20,527,089
Fixed maturities held-for-trading:	326,752	615,839
Short-term investments	2,067,786	134,738
Total derivative instruments	160,914	68,600
Separate account assets	12,058,035	11,381,227
Total assets	34,769,019	32,727,493
Liabilities
Total derivative instruments	70,011	76,373
Separate account liabilities	368,282	290,293
Total liabilities	438,293	366,666
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	14,649	11,843
Liabilities
Total derivative instruments	24,150
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	52,287	28,736
Liabilities
Total derivative instruments	1,432	22
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	33,083	8,295
Liabilities
Total derivative instruments	10,563	5,055
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments	138	189
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments	20,582
Liabilities
Total derivative instruments	5,027
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	40,175	19,537
Liabilities
Total derivative instruments	28,839	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:	1,181,290	3,341,752
Fixed maturities held-for-trading:	268,691	558,208
Recurring basis | Significant other observable inputs (Level 2) | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:	2,273,641	2,219,173
Recurring basis | Significant other observable inputs (Level 2) | Foreign government securities
Assets
Fixed maturities available-for-sale:	2,205	2,286
Recurring basis | Significant other observable inputs (Level 2) | Corporate debt securities
Assets
Fixed maturities available-for-sale:	13,792,661	12,501,174
Fixed maturities held-for-trading:	56,932	56,566
Recurring basis | Significant other observable inputs (Level 2) | Asset-backed securities
Assets
Fixed maturities available-for-sale:	1,680,061	1,311,370
Recurring basis | Significant other observable inputs (Level 2) | Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:	96,158	125,372
Recurring basis | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:	1,090,803	1,016,908
Fixed maturities held-for-trading:	1,129	1,065
Recurring basis | Significant other observable inputs (Level 2) | Collateralized debt obligations
Assets
Fixed maturities available-for-sale:	38,713	9,054
Recurring basis | Significant unobservable inputs (Level 3)
Assets
Fixed maturities available-for-sale:	15,056	4,538
Total assets	15,056	4,538
Liabilities
Embedded derivatives - GLWB	30,687	11,257
Total liabilities	30,687	11,257
Recurring basis | Significant unobservable inputs (Level 3) | Corporate debt securities
Assets
Fixed maturities available-for-sale:	15,056	4,538
Recurring basis | Significant unobservable inputs (Level 3) | Asset-backed securities
Assets
Fixed maturities available-for-sale:	$ 0	$ 0