Investments (Available-For-Sale Securities Disclosure) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	$ 4,874,648	$ 4,546,330
Unrealized Gains	80,702	93,777
Unrealized Losses	(16,057)	(24,853)
Total AFS securities	4,939,293	4,615,254
AFS Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	4,753,759	4,352,514
Unrealized Gains	54,178	78,504
Unrealized Losses	(15,397)	(22,815)
Total AFS securities	4,792,540	4,408,203
AFS Fixed Income Securities [Member] \| U.S. Government and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	75,139	99,485
Unrealized Gains	2,230	4,721
Unrealized Losses	(36)	(91)
Total AFS securities	77,333	104,115
AFS Fixed Income Securities [Member] \| Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	26,559	14,885
Unrealized Gains	322	298
Unrealized Losses	(16)	(2)
Total AFS securities	26,865	15,181
AFS Fixed Income Securities [Member] \| Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	1,366,287	1,314,779
Unrealized Gains	18,610	44,523
Unrealized Losses	(5,304)	(160)
Total AFS securities	1,379,593	1,359,142
AFS Fixed Income Securities [Member] \| Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	1,976,556	1,892,296
Unrealized Gains	27,057	23,407
Unrealized Losses	(5,860)	(15,521)
Total AFS securities	1,997,753	1,900,182
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	527,876	244,541
Unrealized Gains	1,439	531
Unrealized Losses	(355)	(918)
Total AFS securities	528,960	244,154
AFS Fixed Income Securities [Member] \| Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	256,356	245,252
Unrealized Gains	1,514	750
Unrealized Losses	(1,028)	(2,410)
Total AFS securities	256,842	243,592
AFS Fixed Income Securities [Member] \| Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	524,986	541,276
Unrealized Gains	3,006	4,274
Unrealized Losses	(2,798)	(3,713)
Total AFS securities	525,194	541,837
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	120,889	193,816
Unrealized Gains	26,524	15,273
Unrealized Losses	(660)	(2,038)
Total AFS securities	146,753	207,051
Equity Securities [Member] \| Common Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	104,663	181,991
Unrealized Gains	26,250	14,796
Unrealized Losses	(305)	(1,998)
Total AFS securities	130,608	194,789
Equity Securities [Member] \| Nonredeemable Preferred Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	16,226	11,825
Unrealized Gains	274	477
Unrealized Losses	(355)	(40)
Total AFS securities	$ 16,145	$ 12,262

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
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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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