Investments (Gross Unrealized Losses on Investments, By Category and Length of Time) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale, continuous unrealized loss position, less than 12 months, fair value	$ 19,341,518	$ 3,999,294
Available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(803,628)	(47,884)
Available for sale, continuous unrealized loss position, 12 months or more, fair value	6,619,197	4,815,325
Available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(547,459)	(221,017)
Available for sale, continuous unrealized loss position, total, fair value	25,960,715	8,814,619
Available for sale, continuous unrealized loss position, total, unrealized losses	(1,351,087)	(268,901)
United States Government Full Faith and Credit
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale, continuous unrealized loss position, less than 12 months, fair value	543	1,565
Available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(3)	(10)
Available for sale, continuous unrealized loss position, 12 months or more, fair value	7,785	6,731
Available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(319)	(137)
Available for sale, continuous unrealized loss position, total, fair value	8,328	8,296
Available for sale, continuous unrealized loss position, total, unrealized losses	(322)	(147)
United States Government Sponsored Agencies
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale, continuous unrealized loss position, less than 12 months, fair value	30,089	44,794
Available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(949)	(180)
Available for sale, continuous unrealized loss position, 12 months or more, fair value	953,421	958,965
Available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(82,085)	(31,550)
Available for sale, continuous unrealized loss position, total, fair value	983,510	1,003,759
Available for sale, continuous unrealized loss position, total, unrealized losses	(83,034)	(31,730)
United States Municipalities, States and Territories
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale, continuous unrealized loss position, less than 12 months, fair value	340,103	44,736
Available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(6,816)	(128)
Available for sale, continuous unrealized loss position, 12 months or more, fair value	162,997	128,499
Available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(8,842)	(3,468)
Available for sale, continuous unrealized loss position, total, fair value	503,100	173,235
Available for sale, continuous unrealized loss position, total, unrealized losses	(15,658)	(3,596)
Foreign Government Obligations
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale, continuous unrealized loss position, less than 12 months, fair value	98,511	49,663
Available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(1,748)	(337)
Available for sale, continuous unrealized loss position, 12 months or more, fair value	11,859	12,625
Available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(2,411)	(1,688)
Available for sale, continuous unrealized loss position, total, fair value	110,370	62,288
Available for sale, continuous unrealized loss position, total, unrealized losses	(4,159)	(2,025)
Corporate Securities | Finance, Insurance and Real Estate
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale, continuous unrealized loss position, less than 12 months, fair value	2,501,640	456,244
Available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(87,220)	(5,135)
Available for sale, continuous unrealized loss position, 12 months or more, fair value	884,870	600,655
Available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(77,507)	(28,043)
Available for sale, continuous unrealized loss position, total, fair value	3,386,510	1,056,899
Available for sale, continuous unrealized loss position, total, unrealized losses	(164,727)	(33,178)
Corporate Securities | Manufacturing, Construction and Mining
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale, continuous unrealized loss position, less than 12 months, fair value	2,045,859	222,985
Available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(84,972)	(3,475)
Available for sale, continuous unrealized loss position, 12 months or more, fair value	349,738	231,196
Available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(34,635)	(10,849)
Available for sale, continuous unrealized loss position, total, fair value	2,395,597	454,181
Available for sale, continuous unrealized loss position, total, unrealized losses	(119,607)	(14,324)
Corporate Securities | Utilities and Related Sectors
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale, continuous unrealized loss position, less than 12 months, fair value	2,313,271	395,183
Available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(82,119)	(4,099)
Available for sale, continuous unrealized loss position, 12 months or more, fair value	591,482	249,416
Available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(45,838)	(8,901)
Available for sale, continuous unrealized loss position, total, fair value	2,904,753	644,599
Available for sale, continuous unrealized loss position, total, unrealized losses	(127,957)	(13,000)
Corporate Securities | Wholesale/Retail Trade
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale, continuous unrealized loss position, less than 12 months, fair value	1,032,603	152,941
Available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(51,228)	(1,249)
Available for sale, continuous unrealized loss position, 12 months or more, fair value	198,805	178,635
Available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(26,326)	(11,371)
Available for sale, continuous unrealized loss position, total, fair value	1,231,408	331,576
Available for sale, continuous unrealized loss position, total, unrealized losses	(77,554)	(12,620)
Corporate Securities | Services, Media and Other
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale, continuous unrealized loss position, less than 12 months, fair value	4,618,477	729,124
Available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(196,520)	(19,000)
Available for sale, continuous unrealized loss position, 12 months or more, fair value	1,072,722	891,654
Available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(152,364)	(53,565)
Available for sale, continuous unrealized loss position, total, fair value	5,691,199	1,620,778
Available for sale, continuous unrealized loss position, total, unrealized losses	(348,884)	(72,565)
Corporate Securities | Insurance
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position [Abstract]
Held for investment, continuous unrealized loss position, less than 12 months, fair value		0
Held for investment, continuous unrealized loss position, less than 12 months, unrealized losses		0
Held for investment, continuous unrealized loss position, 12 months or more, fair value		76,460
Held for investment, continuous unrealized loss position, 12 months or more, unrealized losses		(581)
Held for investment, continuous unrealized loss position, total, fair value		76,460
Held for investment, continuous unrealized loss position, total, unrealized losses		(581)
Residential Mortgage Backed Securities
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale, continuous unrealized loss position, less than 12 months, fair value	145,613	39,771
Available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(2,638)	(387)
Available for sale, continuous unrealized loss position, 12 months or more, fair value	22,689	32,917
Available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(1,487)	(2,084)
Available for sale, continuous unrealized loss position, total, fair value	168,302	72,688
Available for sale, continuous unrealized loss position, total, unrealized losses	(4,125)	(2,471)
Commercial Mortgage Backed Securities
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale, continuous unrealized loss position, less than 12 months, fair value	2,141,560	1,096,757
Available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(37,150)	(10,385)
Available for sale, continuous unrealized loss position, 12 months or more, fair value	2,090,835	1,306,437
Available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(97,676)	(59,455)
Available for sale, continuous unrealized loss position, total, fair value	4,232,395	2,403,194
Available for sale, continuous unrealized loss position, total, unrealized losses	(134,826)	(69,840)
Other Asset Backed Securities
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale, continuous unrealized loss position, less than 12 months, fair value	4,073,249	765,531
Available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	(252,265)	(3,499)
Available for sale, continuous unrealized loss position, 12 months or more, fair value	271,994	217,595
Available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	(17,969)	(9,906)
Available for sale, continuous unrealized loss position, total, fair value	4,345,243	983,126
Available for sale, continuous unrealized loss position, total, unrealized losses	$ (270,234)	$ (13,405)