Investments (Available-For-Sale Securities Disclosure) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	$ 5,136,597	$ 5,220,527
Unrealized Gains	39,251	130,058
Unrealized Losses	(34,211)	(5,358)
AFS securities, fair value	5,141,637	5,345,227
AFS Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost		5,076,716
Unrealized Gains		90,867
Unrealized Losses		(5,061)
AFS securities, fair value		5,162,522
AFS Fixed Income Securities [Member] \| U.S. Government and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	51,163	49,326
Unrealized Gains	410	647
Unrealized Losses	(624)	(233)
AFS securities, fair value	50,949	49,740
AFS Fixed Income Securities [Member] \| Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	18,032	18,040
Unrealized Gains	253	526
Unrealized Losses	(85)	(11)
AFS securities, fair value	18,200	18,555
AFS Fixed Income Securities [Member] \| Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	1,304,321	1,539,307
Unrealized Gains	18,475	44,245
Unrealized Losses	(4,636)	(582)
AFS securities, fair value	1,318,160	1,582,970
AFS Fixed Income Securities [Member] \| Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	1,624,105	1,588,339
Unrealized Gains	11,033	30,891
Unrealized Losses	(13,416)	(1,762)
AFS securities, fair value	1,621,722	1,617,468
AFS Fixed Income Securities [Member] \| Collateralized Loan Obligations and Other Asset Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	789,717	789,152
Unrealized Gains	5,765	6,508
Unrealized Losses	(999)	(202)
AFS securities, fair value	794,483	795,458
AFS Fixed Income Securities [Member] \| Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	430,894	382,727
Unrealized Gains	404	1,563
Unrealized Losses	(4,715)	(841)
AFS securities, fair value	426,583	383,449
AFS Fixed Income Securities [Member] \| Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	918,365	709,825
Unrealized Gains	2,911	6,487
Unrealized Losses	(9,736)	(1,430)
AFS securities, fair value	$ 911,540	714,882
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost		143,811
Unrealized Gains		39,191
Unrealized Losses		(297)
AFS securities, fair value		182,705
Equity Securities [Member] \| Common Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost		129,696
Unrealized Gains		38,287
Unrealized Losses		(226)
AFS securities, fair value		167,757
Equity Securities [Member] \| Preferred Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost		14,115
Unrealized Gains		904
Unrealized Losses		(71)
AFS securities, fair value		$ 14,948

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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