Investments (Gross Unrealized Losses on Investments, By Category and Length of Time) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value	$ 13,452,495	$ 3,999,294
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(282,536)	(47,884)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	4,695,940	4,815,325
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(341,453)	(221,017)
Available for sale securities, continuous unrealized loss position, total, fair value	18,148,435	8,814,619
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(623,989)	(268,901)
United States Government Full Faith and Credit [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value	3,193	1,565
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(50)	(10)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	6,380	6,731
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(323)	(137)
Available for sale securities, continuous unrealized loss position, total, fair value	9,573	8,296
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(373)	(147)
United States Government Sponsored Agencies [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value	74,632	44,794
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(1,513)	(180)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	932,691	958,965
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(57,827)	(31,550)
Available for sale securities, continuous unrealized loss position, total, fair value	1,007,323	1,003,759
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(59,340)	(31,730)
United States Municipalities, States and Territories [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value	224,928	44,736
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(5,000)	(128)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	124,097	128,499
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(7,855)	(3,468)
Available for sale securities, continuous unrealized loss position, total, fair value	349,025	173,235
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(12,855)	(3,596)
Foreign Government Obligations [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value	127,620	49,663
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(2,442)	(337)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	12,297	12,625
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(2,006)	(1,688)
Available for sale securities, continuous unrealized loss position, total, fair value	139,917	62,288
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(4,448)	(2,025)
Corporate Securities [Member] | Finance, Insurance and Real Estate Sectors [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value	1,980,979	456,244
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(46,008)	(5,135)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	599,683	600,655
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(48,915)	(28,043)
Available for sale securities, continuous unrealized loss position, total, fair value	2,580,662	1,056,899
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(94,923)	(33,178)
Corporate Securities [Member] | Manufacturing, Construction and Mining [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value	1,378,585	222,985
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(29,518)	(3,475)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	229,805	231,196
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(17,840)	(10,849)
Available for sale securities, continuous unrealized loss position, total, fair value	1,608,390	454,181
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(47,358)	(14,324)
Corporate Securities [Member] | Utilities and Related Sectors [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value	1,830,012	395,183
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(41,549)	(4,099)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	239,209	249,416
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(18,121)	(8,901)
Available for sale securities, continuous unrealized loss position, total, fair value	2,069,221	644,599
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(59,670)	(13,000)
Corporate Securities [Member] | Wholesale/Retail Trade [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value	755,307	152,941
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(16,446)	(1,249)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	173,229	178,635
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(16,809)	(11,371)
Available for sale securities, continuous unrealized loss position, total, fair value	928,536	331,576
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(33,255)	(12,620)
Corporate Securities [Member] | Services, Media and Other [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value	3,498,221	729,124
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(85,932)	(19,000)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	855,680	891,654
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(82,692)	(53,565)
Available for sale securities, continuous unrealized loss position, total, fair value	4,353,901	1,620,778
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(168,624)	(72,565)
Corporate Securities [Member] | Insurance [Member]
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Held for investment securities, continuous unrealized loss position, less than 12 months, fair value	0	0
Held for investment securities, continuous unrealized loss position, less than 12 months, unrealized losses	0	0
Held for investment securities, continuous unrealized loss position, 12 months or longer, fair value	69,441	76,460
Held for investment securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(7,602)	(581)
Held for investment securities, continuous unrealized loss position, total, fair value	69,441	76,460
Held for investment securities, continuous unrealized loss position, total, unrealized losses	(7,602)	(581)
Residential Mortgage Backed Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value	286,561	39,771
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(4,535)	(387)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	24,870	32,917
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(2,469)	(2,084)
Available for sale securities, continuous unrealized loss position, total, fair value	311,431	72,688
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(7,004)	(2,471)
Commercial Mortgage Backed Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value	2,495,283	1,096,757
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(43,167)	(10,385)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	1,310,142	1,306,437
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(78,339)	(59,455)
Available for sale securities, continuous unrealized loss position, total, fair value	3,805,425	2,403,194
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(121,506)	(69,840)
Other Asset Backed Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value	797,174	765,531
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(6,376)	(3,499)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	187,857	217,595
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(8,257)	(9,906)
Available for sale securities, continuous unrealized loss position, total, fair value	985,031	983,126
Available for sale securities, continuous unrealized loss position, total, unrealized losses	$ (14,633)	$ (13,405)