Summary of Investments - SOI Unrealized losses fixed maturities AFS (Details 6) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($) position	Dec. 31, 2016 USD ($) position
Fixed maturities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	$ 6,257,198	$ 7,428,519
Less than twelve months, Unrealized loss and OTTI	166,627	235,788
Twelve months or longer, Estimated fair value	1,221,969	1,151,873
Twelve months or longer, Unrealized loss and OTTI	123,664	154,441
Total Estimated fair value	7,479,167	8,580,392
Total Unrealized losses and OTTI	$ 290,291	$ 390,229
Additional disclosure
Total number of securities in an unrealized loss position, Less than twelve months | position	554	610
Total number of securities in an unrealized loss position, twelve months or longer | position	132	128
Total number of securities in an unrealized loss position | position	686	738
Decrease (increase) in total unrealized losses and other-than-temporary impairment losses	$ 99,938
Decrease (increase) in total unrealized losses and other-than-temporary impairment losses (as a percent)	26.00%
Increase (decrease) in the less than twelve months category	$ (69,161)
Increase (decrease) in greater than twelve months unrealized losses and other-than-temporary impairment losses	(30,777)
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	1,254,124	$ 2,006,588
Less than twelve months, Unrealized loss and OTTI	18,974	34,752
Twelve months or longer, Estimated fair value	10,181	10,526
Twelve months or longer, Unrealized loss and OTTI	215	206
Total Estimated fair value	1,264,305	2,017,114
Total Unrealized losses and OTTI	19,189	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	165,820	216,154
Less than twelve months, Unrealized loss and OTTI	4,641	5,922
Twelve months or longer, Estimated fair value	10,394	10,498
Twelve months or longer, Unrealized loss and OTTI	480	395
Total Estimated fair value	176,214	226,652
Total Unrealized losses and OTTI	5,121	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	3,711,201	4,119,630
Less than twelve months, Unrealized loss and OTTI	121,580	170,453
Twelve months or longer, Estimated fair value	880,097	860,153
Twelve months or longer, Unrealized loss and OTTI	107,862	138,711
Total Estimated fair value	4,591,298	4,979,783
Total Unrealized losses and OTTI	$ 229,442	309,164
Additional disclosure
Unrealized losses and OTTI over twelve months on securities (as a percent)	87.00%
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	$ 7,851
Perpetual debt investments
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Twelve months or longer, Unrealized loss and OTTI	2,176
Asset-backed securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	458,552	316,065
Less than twelve months, Unrealized loss and OTTI	4,905	6,971
Twelve months or longer, Estimated fair value	271,982	230,331
Twelve months or longer, Unrealized loss and OTTI	11,766	11,417
Total Estimated fair value	730,534	546,396
Total Unrealized losses and OTTI	$ 16,671	18,388
Additional disclosure
Unrealized losses and OTTI over twelve months on securities (as a percent)	10.00%
Residential mortgage-backed securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	$ 5,755	16,962
Less than twelve months, Unrealized loss and OTTI	18	102
Twelve months or longer, Estimated fair value	13,386	14,297
Twelve months or longer, Unrealized loss and OTTI	978	1,065
Total Estimated fair value	19,141	31,259
Total Unrealized losses and OTTI	996	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	630,766	592,508
Less than twelve months, Unrealized loss and OTTI	16,473	17,535
Twelve months or longer, Estimated fair value	35,929	26,068
Twelve months or longer, Unrealized loss and OTTI	2,363	2,647
Total Estimated fair value	666,695	618,576
Total Unrealized losses and OTTI	18,836	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	30,980	160,612
Less than twelve months, Unrealized loss and OTTI	36	53
Twelve months or longer, Estimated fair value	0	0
Twelve months or longer, Unrealized loss and OTTI	0	0
Total Estimated fair value	30,980	160,612
Total Unrealized losses and OTTI	$ 36	$ 53