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Securities Available-for-Sale - Composition of Securities Available-for-Sale (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost | $ 695,284 |  | $ 686,205 |
Gross Unrealized Gains | 6,975 |  | 10,754 |
Gross Unrealized Losses | (3,968) |  | (3,472) |
Fair Value | 698,291 |  | 693,487 |
Security purchased pending settlement | 5,884 |  | 0 | $ 0
U.S. Treasury securities |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost | 19,943 |  | 24,891 |
Gross Unrealized Gains | 0 |  | 47 |
Gross Unrealized Losses | (154) |  | (127) |
Fair Value | 19,789 |  | 24,811 |
Obligations of U.S. government corporations and agencies |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost | 162,045 |  | 230,989 |
Gross Unrealized Gains | 341 |  | 1,573 |
Gross Unrealized Losses | (193) |  | (383) |
Fair Value | 162,193 |  | 232,179 |
Collateralized mortgage obligations of U.S. government corporations and agencies |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost | 109,916 |  | 130,046 |
Gross Unrealized Gains | 93 |  | 465 |
Gross Unrealized Losses | (1,321) |  | (734) |
Fair Value | 108,688 |  | 129,777 |
Residential mortgage-backed securities of U.S. government corporations and agencies |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost | 32,388 |  | 36,606 |
Gross Unrealized Gains | 679 |  | 984 |
Gross Unrealized Losses | (213) |  | (232) |
Fair Value | 32,854 |  | 37,358 |
Commercial mortgage-backed securities of U.S. government corporations and agencies |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost | 244,018 | [1] | 127,311 |
Gross Unrealized Gains | 247 | [1] | 243 |
Gross Unrealized Losses | (2,044) | [1] | (1,950) |
Fair Value | 242,221 | [1] | 125,604 |
Security purchased pending settlement | 5,900 |  |  |
Obligations of states and political subdivisions |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost | 123,159 |  | 128,783 |
Gross Unrealized Gains | 4,285 |  | 3,772 |
Gross Unrealized Losses | (42) |  | (46) |
Fair Value | 127,402 |  | 132,509 |
Debt Securities |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost | 691,469 |  | 678,626 |
Gross Unrealized Gains | 5,645 |  | 7,084 |
Gross Unrealized Losses | (3,967) |  | (3,472) |
Fair Value | 693,147 |  | 682,238 |
Marketable equity securities |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost | 3,815 |  | 7,579 |
Gross Unrealized Gains | 1,330 |  | 3,670 |
Gross Unrealized Losses | (1) |  | 0 |
Fair Value | $ 5,144 |  | $ 11,249 |
[1] | Includes a $5.9 million security purchase that was pending settlement as of December 31, 2017.

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