Investments (Gross Unrealized Losses on Investments, By Category and Length of Time) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, fair value	$ 5,128,156	$ 19,341,518
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(186,146)	(803,628)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value	9,145,105	6,619,197
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(394,394)	(547,459)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value	14,273,261	25,960,715
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses	(580,540)	(1,351,087)
United States Government Full Faith and Credit
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, fair value	0	543
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	0	(3)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value	7,794	7,785
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(144)	(319)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value	7,794	8,328
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses	(144)	(322)
United States Government Sponsored Agencies
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, fair value	0	30,089
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	0	(949)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value	1,014,518	953,421
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(39,828)	(82,085)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value	1,014,518	983,510
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses	(39,828)	(83,034)
United States Municipalities, States and Territories
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, fair value	13,121	340,103
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(48)	(6,816)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value	130,540	162,997
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(3,531)	(8,842)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value	143,661	503,100
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses	(3,579)	(15,658)
Foreign Government Obligations
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, fair value	0	98,511
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	0	(1,748)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value	37,676	11,859
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(1,651)	(2,411)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value	37,676	110,370
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses	(1,651)	(4,159)
Corporate Securities | Finance, Insurance and Real Estate Sectors
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, fair value	255,189	2,501,640
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(11,742)	(87,220)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value	1,304,783	884,870
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(47,440)	(77,507)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value	1,559,972	3,386,510
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses	(59,182)	(164,727)
Corporate Securities | Manufacturing, Construction and Mining
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, fair value	263,649	2,045,859
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(5,570)	(84,972)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value	879,477	349,738
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(39,948)	(34,635)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value	1,143,126	2,395,597
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses	(45,518)	(119,607)
Corporate Securities | Utilities and Related Sectors
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, fair value	186,483	2,313,271
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(5,154)	(82,119)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value	1,076,726	591,482
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(34,703)	(45,838)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value	1,263,209	2,904,753
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses	(39,857)	(127,957)
Corporate Securities | Wholesale/Retail Trade
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, fair value	117,766	1,032,603
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(2,227)	(51,228)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value	468,303	198,805
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(30,444)	(26,326)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value	586,069	1,231,408
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses	(32,671)	(77,554)
Corporate Securities | Services, Media and Other
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, fair value	436,745	4,618,477
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(15,419)	(196,520)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value	2,154,502	1,072,722
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(131,629)	(152,364)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value	2,591,247	5,691,199
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses	(147,048)	(348,884)
Residential Mortgage Backed Securities
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, fair value	41,759	145,613
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(485)	(2,638)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value	14,201	22,689
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(967)	(1,487)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value	55,960	168,302
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses	(1,452)	(4,125)
Commercial Mortgage Backed Securities
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, fair value	71,872	2,141,560
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(514)	(37,150)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value	1,611,230	2,090,835
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(39,373)	(97,676)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value	1,683,102	4,232,395
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses	(39,887)	(134,826)
Other Asset Backed Securities
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, fair value	3,741,572	4,073,249
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(144,987)	(252,265)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value	445,355	271,994
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(24,736)	(17,969)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value	4,186,927	4,345,243
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses	$ (169,723)	$ (270,234)