Investments (Available-For-Sale Securities Disclosure) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	$ 5,220,527	$ 4,874,648
Unrealized Gains	130,058	80,702
Unrealized Losses	(5,358)	(16,057)
Total AFS securities	5,345,227	4,939,293
AFS Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	5,076,716	4,753,759
Unrealized Gains	90,867	54,178
Unrealized Losses	(5,061)	(15,397)
Total AFS securities	5,162,522	4,792,540
AFS Fixed Income Securities [Member] \| U.S. Government and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	49,326	75,139
Unrealized Gains	647	2,230
Unrealized Losses	(233)	(36)
Total AFS securities	49,740	77,333
AFS Fixed Income Securities [Member] \| Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	18,040	26,559
Unrealized Gains	526	322
Unrealized Losses	(11)	(16)
Total AFS securities	18,555	26,865
AFS Fixed Income Securities [Member] \| Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	1,539,307	1,366,287
Unrealized Gains	44,245	18,610
Unrealized Losses	(582)	(5,304)
Total AFS securities	1,582,970	1,379,593
AFS Fixed Income Securities [Member] \| Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	1,588,339	1,976,556
Unrealized Gains	30,891	27,057
Unrealized Losses	(1,762)	(5,860)
Total AFS securities	1,617,468	1,997,753
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	789,152	527,876
Unrealized Gains	6,508	1,439
Unrealized Losses	(202)	(355)
Total AFS securities	795,458	528,960
AFS Fixed Income Securities [Member] \| Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	382,727	256,356
Unrealized Gains	1,563	1,514
Unrealized Losses	(841)	(1,028)
Total AFS securities	383,449	256,842
AFS Fixed Income Securities [Member] \| Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	709,825	524,986
Unrealized Gains	6,487	3,006
Unrealized Losses	(1,430)	(2,798)
Total AFS securities	714,882	525,194
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	143,811	120,889
Unrealized Gains	39,191	26,524
Unrealized Losses	(297)	(660)
Total AFS securities	182,705	146,753
Equity Securities [Member] \| Common Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	129,696	104,663
Unrealized Gains	38,287	26,250
Unrealized Losses	(226)	(305)
Total AFS securities	167,757	130,608
Equity Securities [Member] \| Nonredeemable Preferred Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	14,115	16,226
Unrealized Gains	904	274
Unrealized Losses	(71)	(355)
Total AFS securities	$ 14,948	$ 16,145

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
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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