Fair Value Measurements - Recurring by FV hierarchy (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Assets
Fixed maturities available-for-sale:	$ 20,838,179	$ 20,531,627
Fixed maturities held-for-trading:	569,611	615,839
Short-term investments	808,515	267,026
Collateral under securities lending agreements	107,654	0
Collateral under derivative counterparty collateral agreements	33,155	50,924
Total derivative instruments	91,534	68,600
Separate account assets	27,239,781	26,631,193
Liabilities
Payable under securities lending agreements	107,654	0
Collateral under derivative counterparty collateral agreements	22,450	19,060
Total derivative instruments	75,637	76,373
Separate account liabilities	27,239,781	26,631,193
Derivative instruments designated as hedges:
Assets
Total derivative instruments	43,859	40,579
Liabilities
Total derivative instruments	6,898	22
Derivative instruments not designated as hedges:
Assets
Total derivative instruments	47,675	28,021
Liabilities
Total derivative instruments	68,739	76,351
Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	21,619	8,295
Liabilities
Total derivative instruments	7,769	5,055
Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments	179	189
Liabilities
Total derivative instruments	0	0
Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments	4,161
Liabilities
Total derivative instruments	1,105
Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	21,716	19,537
Liabilities
Total derivative instruments	59,865	71,296
U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale:	2,395,847	3,341,752
Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:	2,231,778	2,219,173
Foreign government securities
Assets
Fixed maturities available-for-sale:	2,247	2,286
Corporate debt securities
Assets
Fixed maturities available-for-sale:	13,195,651	12,505,712
Asset-backed securities
Assets
Fixed maturities available-for-sale:	1,735,421	1,311,370
Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:	108,382	125,372
Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:	1,159,864	1,016,908
Collateralized debt obligations
Assets
Fixed maturities available-for-sale:	8,989	9,054
Recurring basis
Assets
Fixed maturities available-for-sale:	20,838,179	20,531,627
Fixed maturities held-for-trading:	569,611	615,839
Short-term investments	808,515	267,026
Collateral under securities lending agreements	107,654
Collateral under derivative counterparty collateral agreements	55,605	69,984
Total derivative instruments	91,534	68,600
Separate account assets	27,239,781	26,631,193
Total assets	49,710,879	48,184,269
Liabilities
Payable under securities lending agreements	107,654
Collateral under derivative counterparty collateral agreements	22,450	19,060
Total derivative instruments	75,637	76,373
Embedded derivatives - GLWB	21,707	11,257
Separate account liabilities	256,923	290,317
Total liabilities	484,371	397,007
Recurring basis | Derivative instruments designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	13,867	11,843
Liabilities
Total derivative instruments	2,424
Recurring basis | Derivative instruments designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	29,992	28,736
Liabilities
Total derivative instruments	4,474	22
Recurring basis | Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	21,619	8,295
Liabilities
Total derivative instruments	7,769	5,055
Recurring basis | Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments	179	189
Recurring basis | Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments	4,161
Liabilities
Total derivative instruments	1,105
Recurring basis | Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	21,716	19,537
Liabilities
Total derivative instruments	59,865	71,296
Recurring basis | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale:	2,395,847	3,341,752
Fixed maturities held-for-trading:	511,509	558,208
Recurring basis | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:	2,231,778	2,219,173
Recurring basis | Foreign government securities
Assets
Fixed maturities available-for-sale:	2,247	2,286
Recurring basis | Corporate debt securities
Assets
Fixed maturities available-for-sale:	13,195,651	12,505,712
Fixed maturities held-for-trading:	56,991	56,566
Recurring basis | Asset-backed securities
Assets
Fixed maturities available-for-sale:	1,735,421	1,311,370
Recurring basis | Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:	108,382	125,372
Recurring basis | Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:	1,159,864	1,016,908
Fixed maturities held-for-trading:	1,111	1,065
Recurring basis | Collateralized debt obligations
Assets
Fixed maturities available-for-sale:	8,989	9,054
Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Short-term investments	369,283	132,288
Collateral under securities lending agreements	107,654
Collateral under derivative counterparty collateral agreements	55,605	69,984
Separate account assets	15,170,321	15,249,966
Total assets	15,702,863	15,452,238
Liabilities
Payable under securities lending agreements	107,654
Collateral under derivative counterparty collateral agreements	22,450	19,060
Separate account liabilities	38	24
Total liabilities	130,142	19,084
Recurring basis | Significant other observable inputs (Level 2)
Assets
Fixed maturities available-for-sale:	20,822,637	20,527,089
Fixed maturities held-for-trading:	569,611	615,839
Short-term investments	439,232	134,738
Total derivative instruments	91,534	68,600
Separate account assets	12,069,460	11,381,227
Total assets	33,992,474	32,727,493
Liabilities
Total derivative instruments	75,637	76,373
Separate account liabilities	256,885	290,293
Total liabilities	332,522	366,666
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	13,867	11,843
Liabilities
Total derivative instruments	2,424
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	29,992	28,736
Liabilities
Total derivative instruments	4,474	22
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	21,619	8,295
Liabilities
Total derivative instruments	7,769	5,055
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments	179	189
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments	4,161
Liabilities
Total derivative instruments	1,105
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	21,716	19,537
Liabilities
Total derivative instruments	59,865	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:	2,395,847	3,341,752
Fixed maturities held-for-trading:	511,509	558,208
Recurring basis | Significant other observable inputs (Level 2) | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:	2,231,778	2,219,173
Recurring basis | Significant other observable inputs (Level 2) | Foreign government securities
Assets
Fixed maturities available-for-sale:	2,247	2,286
Recurring basis | Significant other observable inputs (Level 2) | Corporate debt securities
Assets
Fixed maturities available-for-sale:	13,180,109	12,501,174
Fixed maturities held-for-trading:	56,991	56,566
Recurring basis | Significant other observable inputs (Level 2) | Asset-backed securities
Assets
Fixed maturities available-for-sale:	1,735,421	1,311,370
Recurring basis | Significant other observable inputs (Level 2) | Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:	108,382	125,372
Recurring basis | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:	1,159,864	1,016,908
Fixed maturities held-for-trading:	1,111	1,065
Recurring basis | Significant other observable inputs (Level 2) | Collateralized debt obligations
Assets
Fixed maturities available-for-sale:	8,989	9,054
Recurring basis | Significant unobservable inputs (Level 3)
Assets
Fixed maturities available-for-sale:	15,542	4,538
Total assets	15,542	4,538
Liabilities
Embedded derivatives - GLWB	21,707	11,257
Total liabilities	21,707	11,257
Recurring basis | Significant unobservable inputs (Level 3) | Corporate debt securities
Assets
Fixed maturities available-for-sale:	15,542	4,538
Recurring basis | Significant unobservable inputs (Level 3) | Asset-backed securities
Assets
Fixed maturities available-for-sale:	$ 0	$ 0