Summary of Investments - SOI Fixed Maturity AFS and non-credit OTTI in AOCI (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2016	Dec. 31, 2015
Securities available-for-sale
Amortized cost	$ 18,848,230	$ 20,007,462
Fixed maturities available-for-sale:	20,170,588	20,531,627
Fixed maturities
Securities available-for-sale
Amortized cost	18,848,230	20,007,462
Gross unrealized gains	1,520,272	883,519
Gross unrealized losses	197,914	359,354
Fixed maturities available-for-sale:	20,170,588	20,531,627
OTTI (gain) loss included in AOCI	(80,778)	(88,357)
U.S. government direct obligations and U.S. agencies
Securities available-for-sale
Amortized cost	1,090,483	3,291,167
Gross unrealized gains	90,927	55,193
Gross unrealized losses	120	4,608
Fixed maturities available-for-sale:	1,181,290	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	1,929,763	1,988,214
Gross unrealized gains	344,059	238,862
Gross unrealized losses	181	7,903
Fixed maturities available-for-sale:	2,273,641	2,219,173
OTTI (gain) loss included in AOCI	0	50
Foreign government securities
Securities available-for-sale
Amortized cost	2,209	2,291
Gross unrealized gains	0	0
Gross unrealized losses	4	5
Fixed maturities available-for-sale:	2,205	2,286
OTTI (gain) loss included in AOCI	0	0
Corporate debt securities
Securities available-for-sale
Amortized cost	13,095,232	12,388,886
Gross unrealized gains	895,019	437,207
Gross unrealized losses	182,534	320,381
Fixed maturities available-for-sale:	13,807,717	12,505,712
OTTI (gain) loss included in AOCI	(1,672)	(1,810)
Perpetual debt investments
Securities available-for-sale
Amortized cost	135,187	149,062
Fixed maturities available-for-sale:	97,019	116,423
Asset-backed securities
Securities available-for-sale
Amortized cost	1,562,617	1,196,326
Gross unrealized gains	130,135	128,406
Gross unrealized losses	12,691	13,362
Fixed maturities available-for-sale:	1,680,061	1,311,370
OTTI (gain) loss included in AOCI	(79,080)	(86,474)
Residential mortgage-backed securities
Securities available-for-sale
Amortized cost	93,222	122,146
Gross unrealized gains	3,526	4,734
Gross unrealized losses	590	1,508
Fixed maturities available-for-sale:	96,158	125,372
OTTI (gain) loss included in AOCI	(26)	(123)
Commercial mortgage-backed securities
Securities available-for-sale
Amortized cost	1,036,035	1,009,320
Gross unrealized gains	56,508	19,117
Gross unrealized losses	1,740	11,529
Fixed maturities available-for-sale:	1,090,803	1,016,908
OTTI (gain) loss included in AOCI	0	0
Collateralized debt obligations
Securities available-for-sale
Amortized cost	38,669	9,112
Gross unrealized gains	98	0
Gross unrealized losses	54	58
Fixed maturities available-for-sale:	38,713	9,054
OTTI (gain) loss included in AOCI	$ 0	$ 0

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