Summary of Investments - SOI Unrealized losses fixed maturities AFS (Details 6) $ in Thousands	9 Months Ended
	Sep. 30, 2016 USD ($) position	Dec. 31, 2015 USD ($) position
Fixed maturities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	$ 1,366,903	$ 6,725,965
Less than twelve months, Unrealized loss and OTTI	30,443	230,467
Twelve months or longer, Estimated fair value	1,150,850	821,922
Twelve months or longer, Unrealized loss and OTTI	112,172	128,887
Total Estimated fair value	2,517,753	7,547,887
Total Unrealized losses and OTTI	$ 142,615	$ 359,354
Additional disclosure
Total number of securities in an unrealized loss position, Less than twelve months | position	136	558
Total number of securities in an unrealized loss position, twelve months or longer | position	124	106
Total number of securities in an unrealized loss position | position	260	664
Decrease (increase) in total unrealized losses and other-than-temporary impairment losses	$ 216,739
Decrease (increase) in total unrealized losses and other-than-temporary impairment losses (as a percent)	60.00%
Increase (decrease) in the less than twelve months category	$ (200,024)
Increase (decrease) in greater than twelve months unrealized losses and other-than-temporary impairment losses	(16,715)
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	15,444	$ 1,357,822
Less than twelve months, Unrealized loss and OTTI	21	4,101
Twelve months or longer, Estimated fair value	11,155	23,604
Twelve months or longer, Unrealized loss and OTTI	123	507
Total Estimated fair value	26,599	1,381,426
Total Unrealized losses and OTTI	144	4,608
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	8,841	267,581
Less than twelve months, Unrealized loss and OTTI	110	7,903
Twelve months or longer, Estimated fair value	10,656	0
Twelve months or longer, Unrealized loss and OTTI	219	0
Total Estimated fair value	19,497	267,581
Total Unrealized losses and OTTI	329	7,903
Foreign government securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	2,164	2,286
Less than twelve months, Unrealized loss and OTTI	3	5
Twelve months or longer, Estimated fair value	0	0
Twelve months or longer, Unrealized loss and OTTI	0	0
Total Estimated fair value	2,164	2,286
Total Unrealized losses and OTTI	3	5
Corporate debt securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	835,440	4,412,965
Less than twelve months, Unrealized loss and OTTI	26,089	202,874
Twelve months or longer, Estimated fair value	876,692	552,791
Twelve months or longer, Unrealized loss and OTTI	102,659	117,507
Total Estimated fair value	1,712,132	4,965,756
Total Unrealized losses and OTTI	$ 128,748	320,381
Additional disclosure
Unrealized losses and OTTI over twelve months on securities (as a percent)	92.00%
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	$ 13,441
Perpetual debt investments
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Twelve months or longer, Unrealized loss and OTTI	7,572
Asset-backed securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	338,576	247,082
Less than twelve months, Unrealized loss and OTTI	3,289	4,372
Twelve months or longer, Estimated fair value	208,678	182,404
Twelve months or longer, Unrealized loss and OTTI	7,280	8,990
Total Estimated fair value	547,254	429,486
Total Unrealized losses and OTTI	$ 10,569	13,362
Additional disclosure
Unrealized losses and OTTI over twelve months on securities (as a percent)	7.00%
Residential mortgage-backed securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	$ 7,145	0
Less than twelve months, Unrealized loss and OTTI	4	0
Twelve months or longer, Estimated fair value	16,106	18,625
Twelve months or longer, Unrealized loss and OTTI	324	1,508
Total Estimated fair value	23,251	18,625
Total Unrealized losses and OTTI	328	1,508
Commercial mortgage-backed securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	113,741	429,175
Less than twelve months, Unrealized loss and OTTI	889	11,154
Twelve months or longer, Estimated fair value	27,563	44,498
Twelve months or longer, Unrealized loss and OTTI	1,567	375
Total Estimated fair value	141,304	473,673
Total Unrealized losses and OTTI	2,456	11,529
Collateralized debt obligations
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	45,552	9,054
Less than twelve months, Unrealized loss and OTTI	38	58
Twelve months or longer, Estimated fair value	0	0
Twelve months or longer, Unrealized loss and OTTI	0	0
Total Estimated fair value	45,552	9,054
Total Unrealized losses and OTTI	$ 38	$ 58