Investments (Available-For-Sale Securities Disclosure) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	$ 4,678,897	$ 4,546,330
Unrealized Gains	145,476	93,777
Unrealized Losses	(8,116)	(24,853)
Total AFS securities	4,816,257	4,615,254
AFS Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	4,484,719	4,352,514
Unrealized Gains	123,540	78,504
Unrealized Losses	(7,791)	(22,815)
Total AFS securities	4,600,468	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	97,933	99,485
Unrealized Gains	4,686	4,721
Unrealized Losses	(1)	(91)
Total AFS securities	102,618	104,115
AFS Fixed Income Securities [Member] \| Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	13,358	14,885
Unrealized Gains	384	298
Unrealized Losses	0	(2)
Total AFS securities	13,742	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	1,375,776	1,314,779
Unrealized Gains	59,673	44,523
Unrealized Losses	(134)	(160)
Total AFS securities	1,435,315	1,359,142
AFS Fixed Income Securities [Member] \| Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	1,948,423	1,892,296
Unrealized Gains	45,948	23,407
Unrealized Losses	(6,415)	(15,521)
Total AFS securities	1,987,956	1,900,182
AFS Fixed Income Securities [Member] \| Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	244,924	244,541
Unrealized Gains	1,060	531
Unrealized Losses	(167)	(918)
Total AFS securities	245,817	244,154
AFS Fixed Income Securities [Member] \| Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	249,163	245,252
Unrealized Gains	3,938	750
Unrealized Losses	(209)	(2,410)
Total AFS securities	252,892	243,592
AFS Fixed Income Securities [Member] \| Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	555,142	541,276
Unrealized Gains	7,851	4,274
Unrealized Losses	(865)	(3,713)
Total AFS securities	562,128	541,837
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	194,178	193,816
Unrealized Gains	21,936	15,273
Unrealized Losses	(325)	(2,038)
Total AFS securities	215,789	207,051
Equity Securities [Member] \| Common Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	181,396	181,991
Unrealized Gains	21,548	14,796
Unrealized Losses	(325)	(1,998)
Total AFS securities	202,619	194,789
Equity Securities [Member] \| Preferred Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	12,782	11,825
Unrealized Gains	388	477
Unrealized Losses	0	(40)
Total AFS securities	$ 13,170	$ 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
-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
-Publisher FASB
-Name Accounting Standards Codification
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