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Investments (Investments Held in a Loss Position) (Details) - USD ($) $ in Thousands | Mar. 31, 2017 | Dec. 31, 2016
--- | --- | ---
U.S. Treasury obligations [Member] |  |
Investments held in an unrealized loss position |  |
Fair Value | $ 82,630 | $ 79,833
Unrealized Loss | 818 | 911
Less than 12 months, Fair Value | 82,630 | 79,833
Less than 12 months, Unrealized Loss | 818 | 911
More than 12 months, Fair Value | 0 | 0
More than 12 months, Unrealized Loss | 0 | 0
US Government-sponsored Enterprises Debt Securities [Member] |  |
Investments held in an unrealized loss position |  |
Fair Value | 10,507 | 11,746
Unrealized Loss | 161 | 191
Less than 12 months, Fair Value | 10,507 | 11,746
Less than 12 months, Unrealized Loss | 161 | 191
More than 12 months, Fair Value | 0 | 0
More than 12 months, Unrealized Loss | 0 | 0
State and municipal bonds [Member] |  |
Investments held in an unrealized loss position |  |
Fair Value | 167,434 | 224,884
Unrealized Loss | 4,717 | 6,952
Less than 12 months, Fair Value | 160,343 | 219,276
Less than 12 months, Unrealized Loss | 4,132 | 6,444
More than 12 months, Fair Value | 7,091 | 5,608
More than 12 months, Unrealized Loss | 585 | 508
Corporate debt [Member] |  |
Investments held in an unrealized loss position |  |
Fair Value | 416,640 | 469,632
Unrealized Loss | 6,979 | 8,480
Less than 12 months, Fair Value | 382,246 | 424,721
Less than 12 months, Unrealized Loss | 4,361 | 5,662
More than 12 months, Fair Value | 34,394 | 44,911
More than 12 months, Unrealized Loss | 2,618 | 2,818
Residential mortgage-backed securities [Member] |  |
Investments held in an unrealized loss position |  |
Fair Value | 97,495 | 103,680
Unrealized Loss | 1,956 | 2,046
Less than 12 months, Fair Value | 95,381 | 100,542
Less than 12 months, Unrealized Loss | 1,922 | 1,982
More than 12 months, Fair Value | 2,114 | 3,138
More than 12 months, Unrealized Loss | 34 | 64
Agency commercial mortgage-backed securities [Member] |  |
Investments held in an unrealized loss position |  |
Fair Value | 4,571 | 4,579
Unrealized Loss | 102 | 143
Less than 12 months, Fair Value | 4,191 | 4,192
Less than 12 months, Unrealized Loss | 72 | 114
More than 12 months, Fair Value | 380 | 387
More than 12 months, Unrealized Loss | 30 | 29
Other commercial mortgage-backed securities [Member] |  |
Investments held in an unrealized loss position |  |
Fair Value | 10,890 | 9,822
Unrealized Loss | 106 | 137
Less than 12 months, Fair Value | 9,075 | 9,179
Less than 12 months, Unrealized Loss | 85 | 134
More than 12 months, Fair Value | 1,815 | 643
More than 12 months, Unrealized Loss | 21 | 3
Other asset-backed securities [Member] |  |
Investments held in an unrealized loss position |  |
Fair Value | 46,008 | 44,343
Unrealized Loss | 213 | 267
Less than 12 months, Fair Value | 42,018 | 39,079
Less than 12 months, Unrealized Loss | 207 | 256
More than 12 months, Fair Value | 3,990 | 5,264
More than 12 months, Unrealized Loss | 6 | 11
Debt Securities [Member] |  |
Investments held in an unrealized loss position |  |
Fair Value | 836,175 | 948,519
Unrealized Loss | 15,052 | 19,127
Less than 12 months, Fair Value | 786,391 | 888,568
Less than 12 months, Unrealized Loss | 11,758 | 15,694
More than 12 months, Fair Value | 49,784 | 59,951
More than 12 months, Unrealized Loss | $ 3,294 | $ 3,433

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