Investments (Available-For-Sale Securities Disclosure) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	$ 4,929,982	$ 4,874,648
Unrealized Gains	100,175	80,702
Unrealized Losses	(8,234)	(16,057)
Total AFS securities	5,021,923	4,939,293
AFS Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	4,805,368	4,753,759
Unrealized Gains	69,545	54,178
Unrealized Losses	(7,908)	(15,397)
Total AFS securities	4,867,005	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	86,816	75,139
Unrealized Gains	1,566	2,230
Unrealized Losses	(44)	(36)
Total AFS securities	88,338	77,333
AFS Fixed Income Securities [Member] \| Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	31,965	26,559
Unrealized Gains	628	322
Unrealized Losses	0	(16)
Total AFS securities	32,593	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	1,261,191	1,366,287
Unrealized Gains	28,923	18,610
Unrealized Losses	(2,948)	(5,304)
Total AFS securities	1,287,166	1,379,593
AFS Fixed Income Securities [Member] \| Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	1,799,706	1,976,556
Unrealized Gains	30,493	27,057
Unrealized Losses	(2,073)	(5,860)
Total AFS securities	1,828,126	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	675,838	527,876
Unrealized Gains	2,998	1,439
Unrealized Losses	(401)	(355)
Total AFS securities	678,435	528,960
AFS Fixed Income Securities [Member] \| Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	263,217	256,356
Unrealized Gains	1,314	1,514
Unrealized Losses	(775)	(1,028)
Total AFS securities	263,756	256,842
AFS Fixed Income Securities [Member] \| Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	686,635	524,986
Unrealized Gains	3,623	3,006
Unrealized Losses	(1,667)	(2,798)
Total AFS securities	688,591	525,194
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	124,614	120,889
Unrealized Gains	30,630	26,524
Unrealized Losses	(326)	(660)
Total AFS securities	154,918	146,753
Equity Securities [Member] \| Common Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	108,466	104,663
Unrealized Gains	30,150	26,250
Unrealized Losses	(285)	(305)
Total AFS securities	138,331	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	480	274
Unrealized Losses	(41)	(355)
Total AFS securities	$ 16,587	$ 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)
-URI http://asc.fasb.org/extlink&oid=65884222&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
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-Name Accounting Standards Codification
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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