Summary of Investments - SOI Unrealized losses fixed maturities AFS (Details 6) $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($) position	Dec. 31, 2016 USD ($) position
Fixed maturities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	$ 4,107,378	$ 7,428,519
Less than twelve months, Unrealized loss and OTTI	52,758	235,788
Twelve months or longer, Estimated fair value	1,729,368	1,151,873
Twelve months or longer, Unrealized loss and OTTI	99,527	154,441
Total Estimated fair value	5,836,746	8,580,392
Total Unrealized losses and OTTI	$ 152,285	$ 390,229
Additional disclosure
Total number of securities in an unrealized loss position, Less than twelve months | position	397	610
Total number of securities in an unrealized loss position, twelve months or longer | position	169	128
Total number of securities in an unrealized loss position | position	566	738
Decrease (increase) in total unrealized losses and other-than-temporary impairment losses	$ 237,944
Decrease (increase) in total unrealized losses and other-than-temporary impairment losses (as a percent)	61.00%
Increase (decrease) in the less than twelve months category	$ (183,030)
Increase (decrease) in greater than twelve months unrealized losses and other-than-temporary impairment losses	(54,914)
U.S. government direct obligations and U.S. agencies
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	631,467	$ 2,006,588
Less than twelve months, Unrealized loss and OTTI	8,156	34,752
Twelve months or longer, Estimated fair value	11,037	10,526
Twelve months or longer, Unrealized loss and OTTI	177	206
Total Estimated fair value	642,504	2,017,114
Total Unrealized losses and OTTI	8,333	34,958
Obligations of U.S. states and their subdivisions
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	43,583	216,154
Less than twelve months, Unrealized loss and OTTI	726	5,922
Twelve months or longer, Estimated fair value	18,093	10,498
Twelve months or longer, Unrealized loss and OTTI	893	395
Total Estimated fair value	61,676	226,652
Total Unrealized losses and OTTI	1,619	6,317
Corporate debt securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	2,401,845	4,119,630
Less than twelve months, Unrealized loss and OTTI	35,750	170,453
Twelve months or longer, Estimated fair value	1,360,880	860,153
Twelve months or longer, Unrealized loss and OTTI	82,816	138,711
Total Estimated fair value	3,762,725	4,979,783
Total Unrealized losses and OTTI	$ 118,566	309,164
Additional disclosure
Unrealized losses and OTTI over twelve months on securities (as a percent)	83.00%
Corporate debt securities | Non-investment grade
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Twelve months or longer, Unrealized loss and OTTI	$ 5,693
Asset-backed securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	464,858	316,065
Less than twelve months, Unrealized loss and OTTI	2,246	6,971
Twelve months or longer, Estimated fair value	224,740	230,331
Twelve months or longer, Unrealized loss and OTTI	9,927	11,417
Total Estimated fair value	689,598	546,396
Total Unrealized losses and OTTI	$ 12,173	18,388
Additional disclosure
Unrealized losses and OTTI over twelve months on securities (as a percent)	10.00%
Residential mortgage-backed securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	$ 0	16,962
Less than twelve months, Unrealized loss and OTTI	0	102
Twelve months or longer, Estimated fair value	12,383	14,297
Twelve months or longer, Unrealized loss and OTTI	645	1,065
Total Estimated fair value	12,383	31,259
Total Unrealized losses and OTTI	645	1,167
Commercial mortgage-backed securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	488,610	592,508
Less than twelve months, Unrealized loss and OTTI	5,851	17,535
Twelve months or longer, Estimated fair value	102,235	26,068
Twelve months or longer, Unrealized loss and OTTI	5,069	2,647
Total Estimated fair value	590,845	618,576
Total Unrealized losses and OTTI	10,920	20,182
Collateralized debt obligations
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	77,015	160,612
Less than twelve months, Unrealized loss and OTTI	29	53
Twelve months or longer, Estimated fair value	0	0
Twelve months or longer, Unrealized loss and OTTI	0	0
Total Estimated fair value	77,015	160,612
Total Unrealized losses and OTTI	$ 29	$ 53