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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		Jun. 30, 2018	Dec. 31, 2017
Percent Of Securities of Amortized Cost In An Unrealized Unrecognized Loss Position		1.00%	1.00%
AFS Fair Value, Less than 12 months		$ 3,070,757	$ 878,126
AFS Unrealized Losses, Less than 12 months	[1]	(47,981)	(4,961)
AFS Fair Value, 12 months or longer		17,853	35,686
AFS Unrealized Losses, 12 months or longer	[1]	(652)	(397)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		3,088,610	913,812
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]	(48,633)	(5,358)
Held-to-maturity Securities [Member]
Unrealized Unrecognized Loss		300	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		28,005	23,516
AFS Unrealized Losses, Less than 12 months	[1]	(763)	(233)
AFS Fair Value, 12 months or longer		0	250
AFS Unrealized Losses, 12 months or longer	[1]	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		28,005	23,766
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]	(763)	(233)
AFS Fixed Income Securities [Member] | Foreign Government [Member]
AFS Fair Value, Less than 12 months		7,740	1,481
AFS Unrealized Losses, Less than 12 months	[1]	(129)	(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		7,740	1,481
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]	(129)	(11)
AFS Fixed Income Securities [Member] | Obligations of States and Political Subdivisions [Member]
AFS Fair Value, Less than 12 months		342,653	107,514
AFS Unrealized Losses, Less than 12 months	[1]	(4,316)	(422)
AFS Fair Value, 12 months or longer		3,422	14,139
AFS Unrealized Losses, 12 months or longer	[1]	(126)	(160)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		346,075	121,653
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]	(4,442)	(582)
AFS Fixed Income Securities [Member] | Corporate Securities [Member]
AFS Fair Value, Less than 12 months		1,147,312	238,326
AFS Unrealized Losses, Less than 12 months	[1]	(22,989)	(1,744)
AFS Fair Value, 12 months or longer		2,762	3,228
AFS Unrealized Losses, 12 months or longer	[1]	(142)	(18)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		1,150,074	241,554
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]	(23,131)	(1,762)
AFS Fixed Income Securities [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member]
AFS Fair Value, Less than 12 months		427,307	74,977
AFS Unrealized Losses, Less than 12 months	[1]	(1,813)	(196)
AFS Fair Value, 12 months or longer		774	1,655
AFS Unrealized Losses, 12 months or longer	[1]	(4)	(6)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		428,081	76,632
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]	(1,817)	(202)
AFS Fixed Income Securities [Member] | Commercial Mortgage Backed Securities [Member]
AFS Fair Value, Less than 12 months		378,523	154,267
AFS Unrealized Losses, Less than 12 months	[1]	(6,009)	(773)
AFS Fair Value, 12 months or longer		0	5,214
AFS Unrealized Losses, 12 months or longer	[1]	0	(68)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		378,523	159,481
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]	(6,009)	(841)
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities [Member]
AFS Fair Value, Less than 12 months		739,217	269,485
AFS Unrealized Losses, Less than 12 months	[1]	(11,962)	(1,285)
AFS Fair Value, 12 months or longer		10,895	11,200
AFS Unrealized Losses, 12 months or longer	[1]	(380)	(145)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		750,112	280,685
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	[1]	$ (12,342)	(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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