- SOI Fixed Maturity AFS and non-credit OTTI in AOCI (Details) - USD ($) $ in Thousands	Dec. 31, 2017		Dec. 31, 2016
Securities available-for-sale
Amortized cost	$ 22,762,962		$ 21,672,727
Fixed maturities, available-for-sale	23,593,139		22,153,703
Total fixed maturity investments
Securities available-for-sale
Amortized cost	22,762,962		21,672,727
Gross unrealized gains	973,941		871,205
Gross unrealized losses	143,764		390,229
Fixed maturities, available-for-sale	23,593,139		22,153,703
OTTI (gain) loss included in AOCI	(57,893)	[1]	(73,929)	[2]
U.S. government direct obligations and U.S. agencies
Securities available-for-sale
Amortized cost	1,837,748		3,022,279
Gross unrealized gains	41,777		47,791
Gross unrealized losses	7,883		34,958
Fixed maturities, available-for-sale	1,871,642		3,035,112
OTTI (gain) loss included in AOCI	0	[1]	0	[2]
Obligations of U.S. states and their subdivisions
Securities available-for-sale
Amortized cost	1,872,120		1,890,568
Gross unrealized gains	220,507		214,411
Gross unrealized losses	1,655		6,317
Fixed maturities, available-for-sale	2,090,972		2,098,662
OTTI (gain) loss included in AOCI	0	[1]	0	[2]
Corporate debt securities
Securities available-for-sale
Amortized cost	15,234,473	[3]	13,811,597	[4]
Gross unrealized gains	581,991	[3]	477,316	[4]
Gross unrealized losses	110,377	[3]	309,164	[4]
Fixed maturities, available-for-sale	15,706,087	[3]	13,979,749	[4]
OTTI (gain) loss included in AOCI	(1,018)	[1],[3]	(1,488)	[2],[4]
Asset-backed securities
Securities available-for-sale
Amortized cost	1,622,806		1,226,493
Gross unrealized gains	105,301		104,274
Gross unrealized losses	10,131		18,388
Fixed maturities, available-for-sale	1,717,976		1,312,379
OTTI (gain) loss included in AOCI	(56,735)	[1]	(72,464)	[2]
Residential mortgage-backed securities
Securities available-for-sale
Amortized cost	63,187		138,292
Gross unrealized gains	2,446		3,867
Gross unrealized losses	649		1,167
Fixed maturities, available-for-sale	64,984		140,992
OTTI (gain) loss included in AOCI	(140)	[1]	23	[2]
Commercial mortgage-backed securities
Securities available-for-sale
Amortized cost	1,352,906		1,222,257
Gross unrealized gains	17,692		23,207
Gross unrealized losses	12,989		20,182
Fixed maturities, available-for-sale	1,357,609		1,225,282
OTTI (gain) loss included in AOCI	0	[1]	0	[2]
Collateralized debt obligations
Securities available-for-sale
Amortized cost	779,722		361,241
Gross unrealized gains	4,227		339
Gross unrealized losses	80		53
Fixed maturities, available-for-sale	783,869		361,527
OTTI (gain) loss included in AOCI	0	[1]	0	[2]
Perpetual debt investments
Securities available-for-sale
Amortized cost	89,267		135,187
Fixed maturities, available-for-sale	$ 87,348		$ 113,239

[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 $89,267 and estimated fair value of $87,348.

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