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Investments (Fair Value and Gross Pre-Tax Net Unrealized Loss of Securities By Length of Time) (Details) - AFS Fixed Income Securities [Member] - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
AFS Fair Value, Less than 12 Months	$ 1,290,052	$ 516,259
AFS Unrealized Losses, Less than 12 months	(96,447)	(3,379)
AFS Fair Value, 12 months or longer	157,945	230,770
AFS Unrealized Losses, 12 months or longer	(20,464)	(4,359)
AFS Fair Value	1,447,997	747,029
AFS Unrealized Losses	(116,911)	(7,738)
Foreign Government [Member]
AFS Fair Value, Less than 12 Months	4,376	1,416
AFS Unrealized Losses, Less than 12 months	(252)	(2)
AFS Fair Value, 12 months or longer	0	0
AFS Unrealized Losses, 12 months or longer	0	0
AFS Fair Value	4,376	1,416
AFS Unrealized Losses	(252)	(2)
Obligations of States and Political Subdivisions [Member]
AFS Fair Value, Less than 12 Months	70,830	35,838
AFS Unrealized Losses, Less than 12 months	(1,434)	(270)
AFS Fair Value, 12 months or longer	0	0
AFS Unrealized Losses, 12 months or longer	0	0
AFS Fair Value	70,830	35,838
AFS Unrealized Losses	(1,434)	(270)
Corporate Securities [Member]
AFS Fair Value, Less than 12 Months	553,419	84,832
AFS Unrealized Losses, Less than 12 months	(31,677)	(480)
AFS Fair Value, 12 months or longer	8,185	20,182
AFS Unrealized Losses, 12 months or longer	(1,181)	(830)
AFS Fair Value	561,604	105,014
AFS Unrealized Losses	(32,858)	(1,310)
Collateralized Loan Obligations and Other Asset Backed Securities [Member]
AFS Fair Value, Less than 12 Months	445,676	205,191
AFS Unrealized Losses, Less than 12 months	(49,404)	(1,938)
AFS Fair Value, 12 months or longer	136,413	204,385
AFS Unrealized Losses, 12 months or longer	(18,094)	(3,496)
AFS Fair Value	582,089	409,576
AFS Unrealized Losses	(67,498)	(5,434)
Commercial Mortgage Backed Securities [Member]
AFS Fair Value, Less than 12 Months	144,010	62,893
AFS Unrealized Losses, Less than 12 months	(11,119)	(264)
AFS Fair Value, 12 months or longer	11,373	828
AFS Unrealized Losses, 12 months or longer	(1,023)	(3)
AFS Fair Value	155,383	63,721
AFS Unrealized Losses	(12,142)	(267)
Residential Mortgage Backed Securities [Member]
AFS Fair Value, Less than 12 Months	71,741	126,089
AFS Unrealized Losses, Less than 12 months	(2,561)	(425)
AFS Fair Value, 12 months or longer	1,974	5,375
AFS Unrealized Losses, 12 months or longer	(166)	(30)
AFS Fair Value	73,715	131,464
AFS Unrealized Losses	$ (2,727)	$ (455)

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