Summary of Investments - SOI Unrealized losses fixed maturities AFS (Details 6) $ in Thousands	6 Months Ended
	Jun. 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	$ 4,386,150	$ 7,428,519
Less than twelve months, Unrealized loss and OTTI	89,307	235,788
Twelve months or longer, Estimated fair value	1,303,482	1,151,873
Twelve months or longer, Unrealized loss and OTTI	100,021	154,441
Total Estimated fair value	5,689,632	8,580,392
Total Unrealized losses and OTTI	$ 189,328	$ 390,229
Additional disclosure
Total number of securities in an unrealized loss position, Less than twelve months | position	419	610
Total number of securities in an unrealized loss position, twelve months or longer | position	128	128
Total number of securities in an unrealized loss position | position	547	738
Decrease (increase) in total unrealized losses and other-than-temporary impairment losses	$ 200,901
Decrease (increase) in total unrealized losses and other-than-temporary impairment losses (as a percent)	52.00%
Increase (decrease) in the less than twelve months category	$ (146,481)
Increase (decrease) in greater than twelve months unrealized losses and other-than-temporary impairment losses	(54,420)
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	688,677	$ 2,006,588
Less than twelve months, Unrealized loss and OTTI	8,806	34,752
Twelve months or longer, Estimated fair value	9,694	10,526
Twelve months or longer, Unrealized loss and OTTI	192	206
Total Estimated fair value	698,371	2,017,114
Total Unrealized losses and OTTI	8,998	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	59,432	216,154
Less than twelve months, Unrealized loss and OTTI	1,633	5,922
Twelve months or longer, Estimated fair value	10,316	10,498
Twelve months or longer, Unrealized loss and OTTI	562	395
Total Estimated fair value	69,748	226,652
Total Unrealized losses and OTTI	2,195	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	2,622,456	4,119,630
Less than twelve months, Unrealized loss and OTTI	64,428	170,453
Twelve months or longer, Estimated fair value	973,238	860,153
Twelve months or longer, Unrealized loss and OTTI	86,118	138,711
Total Estimated fair value	3,595,694	4,979,783
Total Unrealized losses and OTTI	$ 150,546	309,164
Additional disclosure
Unrealized losses and OTTI over twelve months on securities (as a percent)	86.00%
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	$ 7,871
Perpetual debt investments
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Twelve months or longer, Unrealized loss and OTTI	1,504
Asset-backed securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	480,642	316,065
Less than twelve months, Unrealized loss and OTTI	4,383	6,971
Twelve months or longer, Estimated fair value	261,342	230,331
Twelve months or longer, Unrealized loss and OTTI	10,375	11,417
Total Estimated fair value	741,984	546,396
Total Unrealized losses and OTTI	$ 14,758	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	12,991	14,297
Twelve months or longer, Unrealized loss and OTTI	797	1,065
Total Estimated fair value	12,991	31,259
Total Unrealized losses and OTTI	797	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	469,738	592,508
Less than twelve months, Unrealized loss and OTTI	10,026	17,535
Twelve months or longer, Estimated fair value	35,901	26,068
Twelve months or longer, Unrealized loss and OTTI	1,977	2,647
Total Estimated fair value	505,639	618,576
Total Unrealized losses and OTTI	12,003	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	65,205	160,612
Less than twelve months, Unrealized loss and OTTI	31	53
Twelve months or longer, Estimated fair value	0	0
Twelve months or longer, Unrealized loss and OTTI	0	0
Total Estimated fair value	65,205	160,612
Total Unrealized losses and OTTI	$ 31	$ 53