Summary of Investments - SOI Fixed Maturity AFS and non-credit OTTI in AOCI (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2015		Dec. 31, 2014
Securities available-for-sale
Total fixed maturities	$ 17,640,135		$ 18,953,144
Fixed maturities available-for-sale:	18,494,354		20,162,078
Fixed maturities
Securities available-for-sale
Total fixed maturities	17,640,135		18,953,144
Gross unrealized gains	1,084,649		1,310,377
Gross unrealized losses	230,430		101,443
Fixed maturities available-for-sale:	18,494,354		20,162,078
OTTI (gain) loss included in AOCI	(92,907)	[1]	(99,016)	[2]
U.S. government direct obligations and U.S. agencies
Securities available-for-sale
Total fixed maturities	1,024,373		3,478,153
Gross unrealized gains	62,312		70,597
Gross unrealized losses	855		1,494
Fixed maturities available-for-sale:	1,085,830		3,547,256
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	2,041,014		1,885,715
Gross unrealized gains	261,202		287,668
Gross unrealized losses	13,915		899
Fixed maturities available-for-sale:	2,288,301		2,172,484
OTTI (gain) loss included in AOCI	0	[1]	0	[2]
Foreign government securities
Securities available-for-sale
Total fixed maturities	2,332		2,455
Gross unrealized gains	0		0
Gross unrealized losses	0		4
Fixed maturities available-for-sale:	2,332		2,451
OTTI (gain) loss included in AOCI	0	[1]	0	[2]
Corporate debt securities
Securities available-for-sale
Total fixed maturities	12,245,679	[3]	11,258,517	[4]
Gross unrealized gains	579,837	[3]	763,036	[4]
Gross unrealized losses	199,040	[3]	82,104	[4]
Fixed maturities available-for-sale:	12,626,476	[3]	11,939,449	[4]
OTTI (gain) loss included in AOCI	(1,915)	[1],[3]	(2,228)	[2],[4]
Perpetual debt investments
Securities available-for-sale
Total fixed maturities	149,062		157,742
Fixed maturities available-for-sale:	119,151		131,799
Asset-backed securities
Securities available-for-sale
Total fixed maturities	1,216,655		1,263,089
Gross unrealized gains	143,264		149,152
Gross unrealized losses	10,150		13,702
Fixed maturities available-for-sale:	1,349,769		1,398,539
OTTI (gain) loss included in AOCI	(90,867)	[1]	(96,603)	[2]
Residential mortgage-backed securities
Securities available-for-sale
Total fixed maturities	132,881		167,793
Gross unrealized gains	5,445		7,368
Gross unrealized losses	1,416		1,932
Fixed maturities available-for-sale:	136,910		173,229
OTTI (gain) loss included in AOCI	(125)	[1]	(185)	[2]
Commercial mortgage-backed securities
Securities available-for-sale
Total fixed maturities	967,885		886,748
Gross unrealized gains	32,540		32,556
Gross unrealized losses	5,054		1,099
Fixed maturities available-for-sale:	995,371		918,205
OTTI (gain) loss included in AOCI	0	[1]	0	[2]
Collateralized debt obligations
Securities available-for-sale
Total fixed maturities	9,316		10,674
Gross unrealized gains	49		0
Gross unrealized losses	0		209
Fixed maturities available-for-sale:	9,365		10,465
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 $149,062 and estimated fair value of $119,151.

[4] Includes perpetual debt investments with amortized cost of $157,742 and estimated fair value of $131,799.