SOI Fixed Maturity AFS and non-credit OTTI in AOCI (Details) - USD ($) $ in Thousands		Dec. 31, 2016		Dec. 31, 2015
Securities available-for-sale
Total fixed maturities		$ 21,672,727		$ 20,007,462
Fixed maturities, available-for-sale		22,153,703		20,531,627
Total fixed maturity investments
Securities available-for-sale
Total fixed maturities		21,672,727		20,007,462
Gross unrealized gains		871,205		883,519
Gross unrealized losses		390,229		359,354
Fixed maturities, available-for-sale		22,153,703		20,531,627
OTTI (gain) loss included in AOCI		(73,929)	[1]	(88,357)	[2]
U.S. government direct obligations and U.S. agencies
Securities available-for-sale
Total fixed maturities		3,022,279		3,291,167
Gross unrealized gains		47,791		55,193
Gross unrealized losses		34,958		4,608
Fixed maturities, available-for-sale		3,035,112		3,341,752
OTTI (gain) loss included in AOCI		0	[1]	0	[2]
Obligations of U.S. states and their subdivisions
Securities available-for-sale
Total fixed maturities		1,890,568		1,988,214
Gross unrealized gains		214,411		238,862
Gross unrealized losses		6,317		7,903
Fixed maturities, available-for-sale		2,098,662		2,219,173
OTTI (gain) loss included in AOCI		0	[1]	50	[2]
Foreign government securities
Securities available-for-sale
Total fixed maturities				2,291
Gross unrealized gains				0
Gross unrealized losses				5
Fixed maturities, available-for-sale				2,286
OTTI (gain) loss included in AOCI	[2]			0
Corporate debt securities
Securities available-for-sale
Total fixed maturities		13,811,597	[3]	12,388,886	[4]
Gross unrealized gains		477,316	[3]	437,207	[4]
Gross unrealized losses		309,164	[3]	320,381	[4]
Fixed maturities, available-for-sale		13,979,749	[3]	12,505,712	[4]
OTTI (gain) loss included in AOCI		(1,488)	[1],[3]	(1,810)	[2],[4]
Perpetual debt investments
Securities available-for-sale
Total fixed maturities		135,187		149,062
Fixed maturities, available-for-sale		113,239		116,423
Asset-backed securities
Securities available-for-sale
Total fixed maturities		1,226,493		1,196,326
Gross unrealized gains		104,274		128,406
Gross unrealized losses		18,388		13,362
Fixed maturities, available-for-sale		1,312,379		1,311,370
OTTI (gain) loss included in AOCI		(72,464)	[1]	(86,474)	[2]
Residential mortgage-backed securities
Securities available-for-sale
Total fixed maturities		138,292		122,146
Gross unrealized gains		3,867		4,734
Gross unrealized losses		1,167		1,508
Fixed maturities, available-for-sale		140,992		125,372
OTTI (gain) loss included in AOCI		23	[1]	(123)	[2]
Commercial mortgage-backed securities
Securities available-for-sale
Total fixed maturities		1,222,257		1,009,320
Gross unrealized gains		23,207		19,117
Gross unrealized losses		20,182		11,529
Fixed maturities, available-for-sale		1,225,282		1,016,908
OTTI (gain) loss included in AOCI		0	[1]	0	[2]
Collateralized debt obligations
Securities available-for-sale
Total fixed maturities		361,241		9,112
Gross unrealized gains		339		0
Gross unrealized losses		53		58
Fixed maturities, available-for-sale		361,527		9,054
OTTI (gain) loss included in AOCI		$ 0	[1]	$ 0	[2]

[1] Indicates the amount of any OTTI (gain) loss included in AOCI that is included in gross unrealized gains and losses. OTTI (gain) loss included in AOCI, as presented above, includes both the initial recognition of non-credit losses and the effects of subsequent increases and decreases in estimated fair value for those fixed maturity securities with previous non-credit impairment. The non-credit loss component of OTTI (gain) loss was in an unrealized gain position due to increases in estimated fair value subsequent to initial recognition of non-credit losses on such securities.

[2] Indicates the amount of any OTTI (gain) loss included in AOCI that is included in gross unrealized gains and losses. OTTI (gain) loss included in AOCI, as presented above, includes both the initial recognition of non-credit losses and the effects of subsequent increases and decreases in estimated fair value for those fixed maturity securities with previous non-credit impairment. The non-credit loss component of OTTI (gain) loss was in an unrealized gain position due to increases in estimated fair value subsequent to initial recognition of non-credit losses on such securities.

[3] Includes perpetual debt investments with amortized cost of $135,187 and estimated fair value of $113,239.

[4] Includes perpetual debt investments with amortized cost of $149,062 and estimated fair value of $116,423.