Summary of Investments - Unrealized losses fixed maturities AFS (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($) position	Dec. 31, 2017 USD ($) position
Fixed maturities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	$ 11,580,133	$ 3,909,634
Less than twelve months, Unrealized loss and OTTI	345,827	29,472
Twelve months or longer, Estimated fair value	2,278,938	2,855,703
Twelve months or longer, Unrealized loss and OTTI	179,995	114,292
Total Estimated fair value	13,859,071	6,765,337
Total Unrealized losses and OTTI	$ 525,822	$ 143,764
Additional disclosure
Total number of securities in an unrealized loss position, Less than twelve months | position	1,076	368
Total number of securities in an unrealized loss position, twelve months or longer | position	268	293
Total number of securities in an unrealized loss position | position	1,344	661
Decrease (increase) in total unrealized losses and other-than-temporary impairment losses	$ (382,058)
Decrease (increase) in total unrealized losses and other-than-temporary impairment losses (as a percent)	(266.00%)
Increase (decrease) in the less than twelve months category	$ 316,355
Increase (decrease) in greater than twelve months unrealized losses and other-than-temporary impairment losses	65,703
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	833,130	$ 755,861
Less than twelve months, Unrealized loss and OTTI	25,923	4,159
Twelve months or longer, Estimated fair value	150,012	230,447
Twelve months or longer, Unrealized loss and OTTI	6,459	3,724
Total Estimated fair value	983,142	986,308
Total Unrealized losses and OTTI	32,382	7,883
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	157,066	24,908
Less than twelve months, Unrealized loss and OTTI	1,958	180
Twelve months or longer, Estimated fair value	36,003	37,012
Twelve months or longer, Unrealized loss and OTTI	2,410	1,475
Total Estimated fair value	193,069	61,920
Total Unrealized losses and OTTI	4,368	1,655
Corporate debt securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	8,687,225	2,229,585
Less than twelve months, Unrealized loss and OTTI	278,236	19,568
Twelve months or longer, Estimated fair value	1,591,254	2,036,323
Twelve months or longer, Unrealized loss and OTTI	142,045	90,809
Total Estimated fair value	10,278,479	4,265,908
Total Unrealized losses and OTTI	$ 420,281	110,377
Additional disclosure
Unrealized losses and OTTI over twelve months on securities (as a percent)	79.00%
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	$ 6,676
Asset-backed securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	764,102	544,778
Less than twelve months, Unrealized loss and OTTI	14,417	3,011
Twelve months or longer, Estimated fair value	211,254	245,341
Twelve months or longer, Unrealized loss and OTTI	9,081	7,120
Total Estimated fair value	975,356	790,119
Total Unrealized losses and OTTI	$ 23,498	10,131
Additional disclosure
Unrealized losses and OTTI over twelve months on securities (as a percent)	16.00%
Residential mortgage-backed securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	$ 3,679	4,405
Less than twelve months, Unrealized loss and OTTI	63	23
Twelve months or longer, Estimated fair value	10,029	11,416
Twelve months or longer, Unrealized loss and OTTI	823	626
Total Estimated fair value	13,708	15,821
Total Unrealized losses and OTTI	886	649
Commercial mortgage-backed securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	838,617	342,820
Less than twelve months, Unrealized loss and OTTI	24,039	2,451
Twelve months or longer, Estimated fair value	280,386	295,164
Twelve months or longer, Unrealized loss and OTTI	19,177	10,538
Total Estimated fair value	1,119,003	637,984
Total Unrealized losses and OTTI	43,216	12,989
Collateralized debt obligations
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	296,314	7,277
Less than twelve months, Unrealized loss and OTTI	1,191	80
Twelve months or longer, Estimated fair value	0	0
Twelve months or longer, Unrealized loss and OTTI	0	0
Total Estimated fair value	296,314	7,277
Total Unrealized losses and OTTI	$ 1,191	$ 80