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Investments (Fair Value and Gross Pre-Tax Net Unrealized Loss of Securities By Length of Time) (Details) - Available-for-sale Securities - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
AFS Fair Value, Less than 12 Months	$ 812,132	$ 415,506
AFS Unrealized Losses, Less than 12 months	(16,207)	(6,108)
AFS Fair Value, 12 months or longer	143,129	185,335
AFS Unrealized Losses, 12 months or longer	(3,922)	(5,434)
AFS Fair Value	955,261	600,841
AFS Unrealized Losses	(20,129)	(11,542)
U.S. Government and Government Agencies [Member]
AFS Fair Value, Less than 12 Months	53,796	11,519
AFS Unrealized Losses, Less than 12 months	(2,104)	(137)
AFS Fair Value, 12 months or longer	0	0
AFS Unrealized Losses, 12 months or longer	0	0
AFS Fair Value	53,796	11,519
AFS Unrealized Losses	(2,104)	(137)
Foreign Government [Member]
AFS Fair Value, Less than 12 Months	3,313	1,122
AFS Unrealized Losses, Less than 12 months	(136)	(3)
AFS Fair Value, 12 months or longer	0	0
AFS Unrealized Losses, 12 months or longer	0	0
AFS Fair Value	3,313	1,122
AFS Unrealized Losses	(136)	(3)
Obligations of States and Political Subdivisions [Member]
AFS Fair Value, Less than 12 Months	18,838	2,223
AFS Unrealized Losses, Less than 12 months	(352)	(11)
AFS Fair Value, 12 months or longer	369	0
AFS Unrealized Losses, 12 months or longer	(8)	0
AFS Fair Value	19,207	2,223
AFS Unrealized Losses	(360)	(11)
Corporate Securities [Member]
AFS Fair Value, Less than 12 Months	306,052	65,187
AFS Unrealized Losses, Less than 12 months	(8,560)	(2,152)
AFS Fair Value, 12 months or longer	14,025	2,400
AFS Unrealized Losses, 12 months or longer	(1,310)	(188)
AFS Fair Value	320,077	67,587
AFS Unrealized Losses	(9,870)	(2,340)
Collateralized Loan Obligations and Other Asset Backed Securities [Member]
AFS Fair Value, Less than 12 Months	282,939	261,746
AFS Unrealized Losses, Less than 12 months	(1,737)	(2,995)
AFS Fair Value, 12 months or longer	105,261	165,661
AFS Unrealized Losses, 12 months or longer	(2,315)	(4,848)
AFS Fair Value	388,200	427,407
AFS Unrealized Losses	(4,052)	(7,843)
Residential Mortgage Backed Securities [Member]
AFS Fair Value, Less than 12 Months	76,684	18,227
AFS Unrealized Losses, Less than 12 months	(1,229)	(194)
AFS Fair Value, 12 months or longer	0	1,181
AFS Unrealized Losses, 12 months or longer	0	(7)
AFS Fair Value	76,684	19,408
AFS Unrealized Losses	(1,229)	(201)
Commercial Mortgage Backed Securities [Member]
AFS Fair Value, Less than 12 Months	70,510	55,482
AFS Unrealized Losses, Less than 12 months	(2,089)	(616)
AFS Fair Value, 12 months or longer	23,474	16,093
AFS Unrealized Losses, 12 months or longer	(289)	(391)
AFS Fair Value	93,984	71,575
AFS Unrealized Losses	$ (2,378)	$ (1,007)

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