| **Investment Securities - Investments' Gross Unrealized Losses and Fair Value Aggregated by Investment Category and Length of Time in Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands** | Mar. 31, 2019 | Dec. 31, 2018 |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Fair Value**                                                                                                                                                                                                 |          |          |
| Less than 12 Months                                                                                                                                                                                            | $ 3,761       | $ 432,522     |
| 12 Months or Longer                                                                                                                                                                                            | 342,311       | 255,149       |
| Total                                                                                                                                                                                                          | 346,072       | 687,671       |
| **Gross Unrealized Losses**                                                                                                                                                                                    |          |          |
| Less than 12 Months                                                                                                                                                                                            | 2             | 6,361         |
| 12 Months or Longer                                                                                                                                                                                            | 3,670         | 7,892         |
| Total                                                                                                                                                                                                          | 3,672         | 14,253        |
| **Fair Value**                                                                                                                                                                                                 |          |          |
| Less than 12 Months                                                                                                                                                                                            | 789           | 117,780       |
| 12 Months or Longer                                                                                                                                                                                            | 141,865       | 127,126       |
| Total                                                                                                                                                                                                          | 142,654       | 244,906       |
| **Gross Unrealized Losses**                                                                                                                                                                                    |          |          |
| Less than 12 Months                                                                                                                                                                                            | 0             | 1,245         |
| 12 Months or Longer                                                                                                                                                                                            | 1,704         | 3,496         |
| Total                                                                                                                                                                                                          | 1,704         | 4,741         |
| Less than 12 Months, Fair Value                                                                                                                                                                                | 4,550         | 550,302       |
| 12 Months or Longer, Fair Value                                                                                                                                                                                | 484,176       | 382,275       |
| Total, Fair Value                                                                                                                                                                                              | 488,726       | 932,577       |
| Less than 12 Months, Gross Unrealized Losses                                                                                                                                                                   | 2             | 7,606         |
| 12 Months or Longer, Gross Unrealized Losses                                                                                                                                                                   | 5,374         | 11,388        |
| Total, Gross Unrealized Losses                                                                                                                                                                                 | 5,376         | 18,994        |
| U.S. Government-sponsored agency securities                                                                                                                                                                    |          |          |
| **Fair Value**                                                                                                                                                                                                 |          |          |
| Less than 12 Months                                                                                                                                                                                            | 0             | 1,490         |
| 12 Months or Longer                                                                                                                                                                                            | 373           | 0             |
| Total                                                                                                                                                                                                          | 373           | 1,490         |
| **Gross Unrealized Losses**                                                                                                                                                                                    |          |          |
| Less than 12 Months                                                                                                                                                                                            | 0             | 3             |
| 12 Months or Longer                                                                                                                                                                                            | 1             | 0             |
| Total                                                                                                                                                                                                          | 1             | 3             |
| **Fair Value**                                                                                                                                                                                                 |          |          |
| Less than 12 Months                                                                                                                                                                                            | 0             | 0             |
| 12 Months or Longer                                                                                                                                                                                            | 22,269        | 22,073        |
| Total                                                                                                                                                                                                          | 22,269        | 22,073        |
| **Gross Unrealized Losses**                                                                                                                                                                                    |          |          |
| Less than 12 Months                                                                                                                                                                                            | 0             | 0             |
| 12 Months or Longer                                                                                                                                                                                            | 350           | 545           |
| Total                                                                                                                                                                                                          | 350           | 545           |
| State and municipal                                                                                                                                                                                            |          |          |
| **Fair Value**                                                                                                                                                                                                 |          |          |
| Less than 12 Months                                                                                                                                                                                            | 0             | 234,431       |
| 12 Months or Longer                                                                                                                                                                                            | 31,483        | 38,028        |
| Total                                                                                                                                                                                                          | 31,483        | 272,459       |
| **Gross Unrealized Losses**                                                                                                                                                                                    |          |          |
| Less than 12 Months                                                                                                                                                                                            | 0             | 3,958         |
| 12 Months or Longer                                                                                                                                                                                            | 532           | 1,896         |
| Total                                                                                                                                                                                                          | 532           | 5,854         |
| **Fair Value**                                                                                                                                                                                                 |          |          |
| Less than 12 Months                                                                                                                                                                                            | 310           | 14,952        |
| 12 Months or Longer                                                                                                                                                                                            | 13,200        | 16,786        |
| Total                                                                                                                                                                                                          | 13,510        | 31,738        |
| **Gross Unrealized Losses**                                                                                                                                                                                    |          |          |
| Less than 12 Months                                                                                                                                                                                            | 0             | 369           |
| 12 Months or Longer                                                                                                                                                                                            | 117           | 503           |
| Total                                                                                                                                                                                                          | 117           | 872           |
| U.S. Government-sponsored mortgage-backed securities                                                                                                                                                           |          |          |
| **Fair Value**                                                                                                                                                                                                 |          |          |
| Less than 12 Months                                                                                                                                                                                            | 3,761         | 196,601       |
| 12 Months or Longer                                                                                                                                                                                            | 310,455       | 217,121       |
| Total                                                                                                                                                                                                          | 314,216       | 413,722       |
| **Gross Unrealized Losses**                                                                                                                                                                                    |          |          |
| Less than 12 Months                                                                                                                                                                                            | 2             | 2,400         |
| 12 Months or Longer                                                                                                                                                                                            | 3,137         | 5,996         |
| Total                                                                                                                                                                                                          | 3,139         | 8,396         |
| **Fair Value**                                                                                                                                                                                                 |          |          |
| Less than 12 Months                                                                                                                                                                                            | 479           | 102,828       |
| 12 Months or Longer                                                                                                                                                                                            | 105,396       | 87,268        |
| Total                                                                                                                                                                                                          | 105,875       | 190,096       |
| **Gross Unrealized Losses**                                                                                                                                                                                    |          |          |
| Less than 12 Months                                                                                                                                                                                            | 0             | 876           |
| 12 Months or Longer                                                                                                                                                                                            | 1,237         | 2,447         |
| Total                                                                                                                                                                                                          | 1,237         | 3,323         |
| Foreign investment                                                                                                                                                                                             |          |          |
| **Fair Value**                                                                                                                                                                                                 |          |          |
| Less than 12 Months                                                                                                                                                                                            | 0             | 0             |
| 12 Months or Longer                                                                                                                                                                                            | 1,000         | 999           |
| Total                                                                                                                                                                                                          | 1,000         | 999           |
| **Gross Unrealized Losses**                                                                                                                                                                                    |          |          |
| Less than 12 Months                                                                                                                                                                                            | 0             | 0             |
| 12 Months or Longer                                                                                                                                                                                            | 0             | 1             |
| Total                                                                                                                                                                                                          | $ 0           | $ 1           |

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months.
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