Summary of Investments - SOI Unrealized losses fixed maturities AFS (Details) $ in Thousands	3 Months Ended
	Mar. 31, 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	$ 8,632,212	$ 3,909,634
Less than twelve months, Unrealized loss and OTTI	187,242	29,472
Twelve months or longer, Estimated fair value	2,363,067	2,855,703
Twelve months or longer, Unrealized loss and OTTI	140,546	114,292
Total Estimated fair value	10,995,279	6,765,337
Total Unrealized losses and OTTI	$ 327,788	$ 143,764
Additional disclosure
Total number of securities in an unrealized loss position, Less than twelve months | position	808	368
Total number of securities in an unrealized loss position, twelve months or longer | position	271	293
Total number of securities in an unrealized loss position | position	1,079	661
Decrease (increase) in total unrealized losses and other-than-temporary impairment losses	$ (184,024)
Decrease (increase) in total unrealized losses and other-than-temporary impairment losses (as a percent)	(128.00%)
Increase (decrease) in the less than twelve months category	$ 157,770
Increase (decrease) in greater than twelve months unrealized losses and other-than-temporary impairment losses	26,254
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	735,249	$ 755,861
Less than twelve months, Unrealized loss and OTTI	19,678	4,159
Twelve months or longer, Estimated fair value	151,508	230,447
Twelve months or longer, Unrealized loss and OTTI	5,913	3,724
Total Estimated fair value	886,757	986,308
Total Unrealized losses and OTTI	25,591	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	94,712	24,908
Less than twelve months, Unrealized loss and OTTI	1,113	180
Twelve months or longer, Estimated fair value	37,455	37,012
Twelve months or longer, Unrealized loss and OTTI	2,113	1,475
Total Estimated fair value	132,167	61,920
Total Unrealized losses and OTTI	3,226	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	6,254,337	2,229,585
Less than twelve months, Unrealized loss and OTTI	140,152	19,568
Twelve months or longer, Estimated fair value	1,667,376	2,036,323
Twelve months or longer, Unrealized loss and OTTI	106,829	90,809
Total Estimated fair value	7,921,713	4,265,908
Total Unrealized losses and OTTI	$ 246,981	110,377
Additional disclosure
Unrealized losses and OTTI over twelve months on securities (as a percent)	76.00%
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	$ 7,612
Asset-backed securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	667,026	544,778
Less than twelve months, Unrealized loss and OTTI	10,231	3,011
Twelve months or longer, Estimated fair value	209,018	245,341
Twelve months or longer, Unrealized loss and OTTI	7,941	7,120
Total Estimated fair value	876,044	790,119
Total Unrealized losses and OTTI	$ 18,172	10,131
Additional disclosure
Unrealized losses and OTTI over twelve months on securities (as a percent)	18.00%
Residential mortgage-backed securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	$ 81,291	4,405
Less than twelve months, Unrealized loss and OTTI	411	23
Twelve months or longer, Estimated fair value	10,464	11,416
Twelve months or longer, Unrealized loss and OTTI	698	626
Total Estimated fair value	91,755	15,821
Total Unrealized losses and OTTI	1,109	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	778,597	342,820
Less than twelve months, Unrealized loss and OTTI	15,656	2,451
Twelve months or longer, Estimated fair value	287,246	295,164
Twelve months or longer, Unrealized loss and OTTI	17,052	10,538
Total Estimated fair value	1,065,843	637,984
Total Unrealized losses and OTTI	32,708	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	21,000	7,277
Less than twelve months, Unrealized loss and OTTI	1	80
Twelve months or longer, Estimated fair value	0	0
Twelve months or longer, Unrealized loss and OTTI	0	0
Total Estimated fair value	21,000	7,277
Total Unrealized losses and OTTI	$ 1	$ 80