Summary of Investments - Unrealized losses fixed maturities AFS (Details) $ in Thousands	9 Months Ended
	Sep. 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	$ 11,488,193	$ 3,909,634
Less than twelve months, Unrealized loss and OTTI	362,724	29,472
Twelve months or longer, Estimated fair value	3,256,738	2,855,703
Twelve months or longer, Unrealized loss and OTTI	236,136	114,292
Total Estimated fair value	14,744,931	6,765,337
Total Unrealized losses and OTTI	$ 598,860	$ 143,764
Additional disclosure
Total number of securities in an unrealized loss position, Less than twelve months | position	1,061	368
Total number of securities in an unrealized loss position, twelve months or longer | position	385	293
Total number of securities in an unrealized loss position | position	1,446	661
Decrease (increase) in total unrealized losses and other-than-temporary impairment losses	$ (455,096)
Decrease (increase) in total unrealized losses and other-than-temporary impairment losses (as a percent)	(317.00%)
Increase (decrease) in the less than twelve months category	$ 333,252
Increase (decrease) in greater than twelve months unrealized losses and other-than-temporary impairment losses	121,844
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	821,260	$ 755,861
Less than twelve months, Unrealized loss and OTTI	29,409	4,159
Twelve months or longer, Estimated fair value	251,365	230,447
Twelve months or longer, Unrealized loss and OTTI	13,281	3,724
Total Estimated fair value	1,072,625	986,308
Total Unrealized losses and OTTI	42,690	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	230,553	24,908
Less than twelve months, Unrealized loss and OTTI	4,602	180
Twelve months or longer, Estimated fair value	37,812	37,012
Twelve months or longer, Unrealized loss and OTTI	2,928	1,475
Total Estimated fair value	268,365	61,920
Total Unrealized losses and OTTI	7,530	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	8,692,153	2,229,585
Less than twelve months, Unrealized loss and OTTI	291,447	19,568
Twelve months or longer, Estimated fair value	2,291,736	2,036,323
Twelve months or longer, Unrealized loss and OTTI	184,445	90,809
Total Estimated fair value	10,983,889	4,265,908
Total Unrealized losses and OTTI	$ 475,892	110,377
Additional disclosure
Unrealized losses and OTTI over twelve months on securities (as a percent)	78.00%
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	$ 6,640
Asset-backed securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	649,403	544,778
Less than twelve months, Unrealized loss and OTTI	13,531	3,011
Twelve months or longer, Estimated fair value	299,315	245,341
Twelve months or longer, Unrealized loss and OTTI	10,973	7,120
Total Estimated fair value	948,718	790,119
Total Unrealized losses and OTTI	$ 24,504	10,131
Additional disclosure
Unrealized losses and OTTI over twelve months on securities (as a percent)	15.00%
Residential mortgage-backed securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	$ 3,395	4,405
Less than twelve months, Unrealized loss and OTTI	70	23
Twelve months or longer, Estimated fair value	9,621	11,416
Twelve months or longer, Unrealized loss and OTTI	899	626
Total Estimated fair value	13,016	15,821
Total Unrealized losses and OTTI	969	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	745,307	342,820
Less than twelve months, Unrealized loss and OTTI	22,217	2,451
Twelve months or longer, Estimated fair value	366,889	295,164
Twelve months or longer, Unrealized loss and OTTI	23,610	10,538
Total Estimated fair value	1,112,196	637,984
Total Unrealized losses and OTTI	45,827	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	346,122	7,277
Less than twelve months, Unrealized loss and OTTI	1,448	80
Twelve months or longer, Estimated fair value	0	0
Twelve months or longer, Unrealized loss and OTTI	0	0
Total Estimated fair value	346,122	7,277
Total Unrealized losses and OTTI	$ 1,448	$ 80