- SOI Unrealized losses fixed maturities AFS (Details) $ in Thousands	Dec. 31, 2017 USD ($) position	Dec. 31, 2016 USD ($) position
Total fixed maturity investments
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	$ 3,909,634	$ 7,428,519
Less than twelve months, Unrealized loss and OTTI	29,472	235,788
Twelve months or longer, Estimated fair value	2,855,703	1,151,873
Unrealized loss and OTTI, 12 months or longer	114,292	154,441
Total Estimated fair value	6,765,337	8,580,392
Total Unrealized losses and OTTI	$ 143,764	$ 390,229
Additional disclosure
Total number of securities in an unrealized loss position, Less than twelve months | position	368	610
Total number of securities in an unrealized loss position, twelve months or longer | position	293	128
Total number of securities in an unrealized loss position | position	661	738
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	$ 755,861	$ 2,006,588
Less than twelve months, Unrealized loss and OTTI	4,159	34,752
Twelve months or longer, Estimated fair value	230,447	10,526
Unrealized loss and OTTI, 12 months or longer	3,724	206
Total Estimated fair value	986,308	2,017,114
Total Unrealized losses and OTTI	7,883	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	24,908	216,154
Less than twelve months, Unrealized loss and OTTI	180	5,922
Twelve months or longer, Estimated fair value	37,012	10,498
Unrealized loss and OTTI, 12 months or longer	1,475	395
Total Estimated fair value	61,920	226,652
Total Unrealized losses and OTTI	1,655	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	2,229,585	4,119,630
Less than twelve months, Unrealized loss and OTTI	19,568	170,453
Twelve months or longer, Estimated fair value	2,036,323	860,153
Unrealized loss and OTTI, 12 months or longer	90,809	138,711
Total Estimated fair value	4,265,908	4,979,783
Total Unrealized losses and OTTI	110,377	309,164
Asset-backed securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	544,778	316,065
Less than twelve months, Unrealized loss and OTTI	3,011	6,971
Twelve months or longer, Estimated fair value	245,341	230,331
Unrealized loss and OTTI, 12 months or longer	7,120	11,417
Total Estimated fair value	790,119	546,396
Total Unrealized losses and OTTI	10,131	18,388
Residential mortgage-backed securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	4,405	16,962
Less than twelve months, Unrealized loss and OTTI	23	102
Twelve months or longer, Estimated fair value	11,416	14,297
Unrealized loss and OTTI, 12 months or longer	626	1,065
Total Estimated fair value	15,821	31,259
Total Unrealized losses and OTTI	649	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	342,820	592,508
Less than twelve months, Unrealized loss and OTTI	2,451	17,535
Twelve months or longer, Estimated fair value	295,164	26,068
Unrealized loss and OTTI, 12 months or longer	10,538	2,647
Total Estimated fair value	637,984	618,576
Total Unrealized losses and OTTI	12,989	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	7,277	160,612
Less than twelve months, Unrealized loss and OTTI	80	53
Twelve months or longer, Estimated fair value	0	0
Unrealized loss and OTTI, 12 months or longer	0	0
Total Estimated fair value	7,277	160,612
Total Unrealized losses and OTTI	$ 80	$ 53