Investments (Available-For-Sale Securities Disclosure) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	$ 5,054,468	$ 4,874,648
Unrealized Gains	129,864	80,702
Unrealized Losses	(3,916)	(16,057)
Total AFS securities	5,180,416	4,939,293
AFS Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	4,923,005	4,753,759
Unrealized Gains	98,646	54,178
Unrealized Losses	(2,929)	(15,397)
Total AFS securities	5,018,722	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	66,932	75,139
Unrealized Gains	1,103	2,230
Unrealized Losses	(32)	(36)
Total AFS securities	68,003	77,333
AFS Fixed Income Securities [Member] \| Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	18,157	26,559
Unrealized Gains	557	322
Unrealized Losses	0	(16)
Total AFS securities	18,714	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	1,360,211	1,366,287
Unrealized Gains	45,641	18,610
Unrealized Losses	(1,086)	(5,304)
Total AFS securities	1,404,766	1,379,593
AFS Fixed Income Securities [Member] \| Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	1,782,993	1,976,556
Unrealized Gains	39,041	27,057
Unrealized Losses	(524)	(5,860)
Total AFS securities	1,821,510	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	698,594	527,876
Unrealized Gains	4,108	1,439
Unrealized Losses	(178)	(355)
Total AFS securities	702,524	528,960
AFS Fixed Income Securities [Member] \| Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	277,320	256,356
Unrealized Gains	2,128	1,514
Unrealized Losses	(243)	(1,028)
Total AFS securities	279,205	256,842
AFS Fixed Income Securities [Member] \| Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	718,798	524,986
Unrealized Gains	6,068	3,006
Unrealized Losses	(866)	(2,798)
Total AFS securities	724,000	525,194
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	131,463	120,889
Unrealized Gains	31,218	26,524
Unrealized Losses	(987)	(660)
Total AFS securities	161,694	146,753
Equity Securities [Member] \| Common Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	115,315	104,663
Unrealized Gains	30,108	26,250
Unrealized Losses	(987)	(305)
Total AFS securities	144,436	130,608
Equity Securities [Member] \| Preferred Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	16,148	16,226
Unrealized Gains	1,110	274
Unrealized Losses	0	(355)
Total AFS securities	$ 17,258	$ 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
-Paragraph 1
-Subparagraph (b)
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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