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Investments (Fair Value and Gross Pre-Tax Net Unrealized Unrecognized Loss of Securities By Length of Time) (Details) - USD ($) $ in Thousands		Sep. 30, 2018	Dec. 31, 2017
Percent Of Securities of Amortized Cost In An Unrealized Unrecognized Loss Position		2.00%	1.00%
AFS Fair Value, Less than 12 months		$ 3,159,528	$ 878,126
AFS Unrealized Losses, Less than 12 months	[1]	(50,169)	(4,961)
AFS Fair Value, 12 months or longer		47,125	35,686
AFS Unrealized Losses, 12 months or longer	[1]	(1,792)	(397)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		3,206,653	913,812
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]	(51,961)	(5,358)
Held-to-maturity Securities [Member]
Unrealized Unrecognized Loss		200	100
AFS Fixed Income Securities [Member]
AFS Fair Value, Less than 12 months			869,566
AFS Unrealized Losses, Less than 12 months	[1]		(4,664)
AFS Fair Value, 12 months or longer			35,686
AFS Unrealized Losses, 12 months or longer	[1]		(397)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value			905,252
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]		(5,061)
AFS Fixed Income Securities [Member] | U.S. Government and Government Agencies [Member]
AFS Fair Value, Less than 12 months		26,414	23,516
AFS Unrealized Losses, Less than 12 months	[1]	(843)	(233)
AFS Fair Value, 12 months or longer		8,497	250
AFS Unrealized Losses, 12 months or longer	[1]	(305)	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		34,911	23,766
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]	(1,148)	(233)
AFS Fixed Income Securities [Member] | Foreign Government [Member]
AFS Fair Value, Less than 12 months		7,281	1,481
AFS Unrealized Losses, Less than 12 months	[1]	(120)	(11)
AFS Fair Value, 12 months or longer		0	0
AFS Unrealized Losses, 12 months or longer	[1]	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		7,281	1,481
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]	(120)	(11)
AFS Fixed Income Securities [Member] | Obligations of States and Political Subdivisions [Member]
AFS Fair Value, Less than 12 months		411,980	107,514
AFS Unrealized Losses, Less than 12 months	[1]	(7,008)	(422)
AFS Fair Value, 12 months or longer		6,397	14,139
AFS Unrealized Losses, 12 months or longer	[1]	(253)	(160)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		418,377	121,653
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]	(7,261)	(582)
AFS Fixed Income Securities [Member] | Corporate Securities [Member]
AFS Fair Value, Less than 12 months		1,140,349	238,326
AFS Unrealized Losses, Less than 12 months	[1]	(19,052)	(1,744)
AFS Fair Value, 12 months or longer		6,373	3,228
AFS Unrealized Losses, 12 months or longer	[1]	(250)	(18)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		1,146,722	241,554
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]	(19,302)	(1,762)
AFS Fixed Income Securities [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member]
AFS Fair Value, Less than 12 months		497,809	74,977
AFS Unrealized Losses, Less than 12 months	[1]	(2,479)	(196)
AFS Fair Value, 12 months or longer		4,335	1,655
AFS Unrealized Losses, 12 months or longer	[1]	(61)	(6)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		502,144	76,632
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]	(2,540)	(202)
AFS Fixed Income Securities [Member] | Commercial Mortgage Backed Securities [Member]
AFS Fair Value, Less than 12 months		376,359	154,267
AFS Unrealized Losses, Less than 12 months	[1]	(5,940)	(773)
AFS Fair Value, 12 months or longer		2,340	5,214
AFS Unrealized Losses, 12 months or longer	[1]	(47)	(68)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		378,699	159,481
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]	(5,987)	(841)
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities [Member]
AFS Fair Value, Less than 12 months		699,336	269,485
AFS Unrealized Losses, Less than 12 months	[1]	(14,727)	(1,285)
AFS Fair Value, 12 months or longer		19,183	11,200
AFS Unrealized Losses, 12 months or longer	[1]	(876)	(145)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		718,519	280,685
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]	$ (15,603)	(1,430)
Equity Securities [Member]
AFS Fair Value, Less than 12 months			8,560
AFS Unrealized Losses, Less than 12 months	[1]		(297)
AFS Fair Value, 12 months or longer			0
AFS Unrealized Losses, 12 months or longer	[1]		0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value			8,560
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]		(297)
Equity Securities [Member] | Common Stock [Member]
AFS Fair Value, Less than 12 months			4,727
AFS Unrealized Losses, Less than 12 months	[1]		(226)
AFS Fair Value, 12 months or longer			0
AFS Unrealized Losses, 12 months or longer	[1]		0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value			4,727
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]		(226)
Equity Securities [Member] | Preferred Stock [Member]
AFS Fair Value, Less than 12 months			3,833
AFS Unrealized Losses, Less than 12 months	[1]		(71)
AFS Fair Value, 12 months or longer			0
AFS Unrealized Losses, 12 months or longer	[1]		0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value			3,833
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]		$ (71)
[1] Gross unrealized losses include non-OTTI unrealized amounts and OTTI losses recognized in AOCI.

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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://fasb.org/us-gaap/role/ref/legacyRef
-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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