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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

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