Investments (Available-For-Sale Securities Disclosure) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	$ 5,172,719	$ 4,874,648
Unrealized Gains	139,783	80,702
Unrealized Losses	(3,798)	(16,057)
Total AFS securities	5,308,704	4,939,293
AFS Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	5,026,735	4,753,759
Unrealized Gains	109,135	54,178
Unrealized Losses	(2,438)	(15,397)
Total AFS securities	5,133,432	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	58,820	75,139
Unrealized Gains	906	2,230
Unrealized Losses	(137)	(36)
Total AFS securities	59,589	77,333
AFS Fixed Income Securities [Member] \| Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	18,149	26,559
Unrealized Gains	650	322
Unrealized Losses	0	(16)
Total AFS securities	18,799	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	1,431,282	1,366,287
Unrealized Gains	49,088	18,610
Unrealized Losses	(432)	(5,304)
Total AFS securities	1,479,938	1,379,593
AFS Fixed Income Securities [Member] \| Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	1,753,584	1,976,556
Unrealized Gains	41,095	27,057
Unrealized Losses	(845)	(5,860)
Total AFS securities	1,793,834	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	747,793	527,876
Unrealized Gains	6,305	1,439
Unrealized Losses	(143)	(355)
Total AFS securities	753,955	528,960
AFS Fixed Income Securities [Member] \| Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	306,173	256,356
Unrealized Gains	2,249	1,514
Unrealized Losses	(318)	(1,028)
Total AFS securities	308,104	256,842
AFS Fixed Income Securities [Member] \| Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	710,934	524,986
Unrealized Gains	8,842	3,006
Unrealized Losses	(563)	(2,798)
Total AFS securities	719,213	525,194
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	145,984	120,889
Unrealized Gains	30,648	26,524
Unrealized Losses	(1,360)	(660)
Total AFS securities	175,272	146,753
Equity Securities [Member] \| Common Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	131,869	104,663
Unrealized Gains	29,900	26,250
Unrealized Losses	(1,342)	(305)
Total AFS securities	160,427	130,608
Equity Securities [Member] \| Preferred Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	14,115	16,226
Unrealized Gains	748	274
Unrealized Losses	(18)	(355)
Total AFS securities	$ 14,845	$ 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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