Summary of Investments - SOI Fixed Maturity AFS and non-credit OTTI in AOCI (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2016	Dec. 31, 2015
Securities available-for-sale
Amortized cost	$ 19,884,469	$ 20,007,462
Fixed maturities available-for-sale:	20,838,179	20,531,627
Fixed maturities
Securities available-for-sale
Amortized cost	19,884,469	20,007,462
Gross unrealized gains	1,201,255	883,519
Gross unrealized losses	247,545	359,354
Fixed maturities available-for-sale:	20,838,179	20,531,627
OTTI (gain) loss included in AOCI	(82,842)	(88,357)
U.S. government direct obligations and U.S. agencies
Securities available-for-sale
Amortized cost	2,306,623	3,291,167
Gross unrealized gains	89,965	55,193
Gross unrealized losses	741	4,608
Fixed maturities available-for-sale:	2,395,847	3,341,752
OTTI (gain) loss included in AOCI	0	0
Obligations of U.S. states and their subdivisions
Securities available-for-sale
Amortized cost	1,942,741	1,988,214
Gross unrealized gains	290,203	238,862
Gross unrealized losses	1,166	7,903
Fixed maturities available-for-sale:	2,231,778	2,219,173
OTTI (gain) loss included in AOCI	0	50
Foreign government securities
Securities available-for-sale
Amortized cost	2,250	2,291
Gross unrealized gains	0	0
Gross unrealized losses	3	5
Fixed maturities available-for-sale:	2,247	2,286
OTTI (gain) loss included in AOCI	0	0
Corporate debt securities
Securities available-for-sale
Amortized cost	12,768,568	12,388,886
Gross unrealized gains	652,028	437,207
Gross unrealized losses	224,945	320,381
Fixed maturities available-for-sale:	13,195,651	12,505,712
OTTI (gain) loss included in AOCI	(1,733)	(1,810)
Perpetual debt investments
Securities available-for-sale
Amortized cost	149,062	149,062
Fixed maturities available-for-sale:	107,421	116,423
Asset-backed securities
Securities available-for-sale
Amortized cost	1,627,812	1,196,326
Gross unrealized gains	123,351	128,406
Gross unrealized losses	15,742	13,362
Fixed maturities available-for-sale:	1,735,421	1,311,370
OTTI (gain) loss included in AOCI	(81,066)	(86,474)
Residential mortgage-backed securities
Securities available-for-sale
Amortized cost	105,454	122,146
Gross unrealized gains	3,808	4,734
Gross unrealized losses	880	1,508
Fixed maturities available-for-sale:	108,382	125,372
OTTI (gain) loss included in AOCI	(43)	(123)
Commercial mortgage-backed securities
Securities available-for-sale
Amortized cost	1,122,101	1,009,320
Gross unrealized gains	41,831	19,117
Gross unrealized losses	4,068	11,529
Fixed maturities available-for-sale:	1,159,864	1,016,908
OTTI (gain) loss included in AOCI	0	0
Collateralized debt obligations
Securities available-for-sale
Amortized cost	8,920	9,112
Gross unrealized gains	69	0
Gross unrealized losses	0	58
Fixed maturities available-for-sale:	8,989	9,054
OTTI (gain) loss included in AOCI	$ 0	$ 0

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