Investments (Gross Unrealized Losses on Investments, By Category and Length of Time) (Details) - USD ($) $ in Thousands	Sep. 30, 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	$ 19,762,544	$ 3,999,294
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(519,847)	(47,884)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	5,517,763	4,815,325
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(459,540)	(221,017)
Available for sale securities, continuous unrealized loss position, total, fair value	25,280,307	8,814,619
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(979,387)	(268,901)
United States Government Full Faith and Credit
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value	3,178	1,565
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(98)	(10)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	5,989	6,731
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(547)	(137)
Available for sale securities, continuous unrealized loss position, total, fair value	9,167	8,296
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(645)	(147)
United States Government Sponsored Agencies
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value	43,310	44,794
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(2,770)	(180)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	927,678	958,965
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(92,825)	(31,550)
Available for sale securities, continuous unrealized loss position, total, fair value	970,988	1,003,759
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(95,595)	(31,730)
United States Municipalities, States and Territories
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value	684,699	44,736
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(21,142)	(128)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	123,625	128,499
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(10,480)	(3,468)
Available for sale securities, continuous unrealized loss position, total, fair value	808,324	173,235
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(31,622)	(3,596)
Foreign Government Obligations
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value	106,283	49,663
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(4,031)	(337)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	60,641	12,625
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(3,640)	(1,688)
Available for sale securities, continuous unrealized loss position, total, fair value	166,924	62,288
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(7,671)	(2,025)
Corporate Securities \| Finance, Insurance and Real Estate Sectors
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value	2,825,957	456,244
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(83,452)	(5,135)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	783,568	600,655
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(75,867)	(28,043)
Available for sale securities, continuous unrealized loss position, total, fair value	3,609,525	1,056,899
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(159,319)	(33,178)
Corporate Securities \| Manufacturing, Construction and Mining
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value	2,145,542	222,985
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(70,140)	(3,475)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	294,755	231,196
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(26,064)	(10,849)
Available for sale securities, continuous unrealized loss position, total, fair value	2,440,297	454,181
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(96,204)	(14,324)
Corporate Securities \| Utilities and Related Sectors
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value	2,569,490	395,183
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(87,914)	(4,099)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	383,587	249,416
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(34,057)	(8,901)
Available for sale securities, continuous unrealized loss position, total, fair value	2,953,077	644,599
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(121,971)	(13,000)
Corporate Securities \| Wholesale/Retail Trade
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value	1,026,417	152,941
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(36,525)	(1,249)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	143,605	178,635
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(18,263)	(11,371)
Available for sale securities, continuous unrealized loss position, total, fair value	1,170,022	331,576
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(54,788)	(12,620)
Corporate Securities \| Services, Media and Other
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value	4,472,035	729,124
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(135,921)	(19,000)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	950,004	891,654
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(105,289)	(53,565)
Available for sale securities, continuous unrealized loss position, total, fair value	5,422,039	1,620,778
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(241,210)	(72,565)
Corporate Securities \| Insurance
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	71,113	76,460
Held for investment securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(6,100)	(581)
Held for investment securities, continuous unrealized loss position, total, fair value	71,113	76,460
Held for investment securities, continuous unrealized loss position, total, unrealized losses	(6,100)	(581)
Residential Mortgage Backed Securities
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value	396,096	39,771
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(9,620)	(387)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	41,760	32,917
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(2,057)	(2,084)
Available for sale securities, continuous unrealized loss position, total, fair value	437,856	72,688
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(11,677)	(2,471)
Commercial Mortgage Backed Securities
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value	2,423,197	1,096,757
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(40,541)	(10,385)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	1,610,912	1,306,437
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(78,819)	(59,455)
Available for sale securities, continuous unrealized loss position, total, fair value	4,034,109	2,403,194
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(119,360)	(69,840)
Other Asset Backed Securities
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value	3,066,340	765,531
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(27,693)	(3,499)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	191,639	217,595
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(11,632)	(9,906)
Available for sale securities, continuous unrealized loss position, total, fair value	3,257,979	983,126
Available for sale securities, continuous unrealized loss position, total, unrealized losses	$ (39,325)	$ (13,405)

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

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-Publisher FASB
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Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity in a continuous loss position.

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