Investments (Gross Unrealized Losses on Investments, By Category and Length of Time) (Details) - USD ($) $ in Thousands	Sep. 30, 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	$ 3,042,097	$ 19,341,518
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(69,087)	(803,628)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value	3,606,209	6,619,197
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(237,944)	(547,459)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value	6,648,306	25,960,715
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses	(307,031)	(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		543
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(3)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		7,785
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		(319)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		8,328
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(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	119,718	30,089
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(282)	(949)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value	203,230	953,421
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(1,770)	(82,085)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value	322,948	983,510
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses	(2,052)	(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	97,111	340,103
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(668)	(6,816)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value	3,116	162,997
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(46)	(8,842)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value	100,227	503,100
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses	(714)	(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		98,511
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(1,748)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		11,859
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		(2,411)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		110,370
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(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	330,617	2,501,640
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(3,576)	(87,220)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value	144,633	884,870
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(2,095)	(77,507)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value	475,250	3,386,510
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses	(5,671)	(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	135,697	2,045,859
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(1,003)	(84,972)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value	56,550	349,738
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(3,921)	(34,635)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value	192,247	2,395,597
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses	(4,924)	(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	56,274	2,313,271
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(2,272)	(82,119)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value	50,072	591,482
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(4,928)	(45,838)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value	106,346	2,904,753
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses	(7,200)	(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	18,356	1,032,603
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(293)	(51,228)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value	121,902	198,805
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(13,234)	(26,326)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value	140,258	1,231,408
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses	(13,527)	(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	188,946	4,618,477
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(10,486)	(196,520)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value	424,760	1,072,722
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(63,140)	(152,364)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value	613,706	5,691,199
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses	(73,626)	(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	116,116	145,613
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(614)	(2,638)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value	11,918	22,689
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(994)	(1,487)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value	128,034	168,302
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses	(1,608)	(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	239,503	2,141,560
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(1,355)	(37,150)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value	203,268	2,090,835
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(3,887)	(97,676)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value	442,771	4,232,395
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses	(5,242)	(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	1,739,759	4,073,249
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(48,538)	(252,265)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value	2,386,760	271,994
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(143,929)	(17,969)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value	4,126,519	4,345,243
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses	$ (192,467)	$ (270,234)