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Securities - Additional Information (Details) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 USD ($) | Sep. 30, 2017 USD ($) | Dec. 31, 2017 USD ($) institution Tranche | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Transferred security, at carrying value |  | $ 365,941,000 |  |  |
Transferred to AFS securities, unrealized gain (loss) |  | $ 13,219,000 |  |  |
Proceeds from sales of securities available for sale |  |  | $ 495,340,000 | $ 4,028,000 | $ 8,444,000
Carrying Value | $ 495,192,000 |  | 495,192,000 |  |
Gross realized gain (loss), excluding other than temporary impairments |  |  | 148,000 |  |
Carrying value | 674,104,000 |  | 674,104,000 |  |
Securities available for sale, at fair value | 671,488,000 |  | $ 671,488,000 | 674,248,000 |
Number of securities representing interests in tranches of trusts (tranches) | Tranche |  |  | 2 |  |
Number of institutions issuing debt (institutions) | institution |  |  | 160 |  |
Trust preferred securities |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Carrying value | 12,442,000 |  | $ 12,442,000 | 23,749,000 |
Securities available for sale, at fair value | 9,388,000 |  | 9,388,000 | 18,389,000 |
Other than temporary impairment losses |  |  | 0 | 0 | 0
Secure government, public and trust deposits |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Available for sale securities pledged as collateral | 217,867,000 |  | 217,867,000 | 642,447,000 |
Short-term borrowings |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Available for sale securities pledged as collateral | 25,888,000 |  | 25,888,000 | 24,426,000 |
Metropolitan Securities |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Proceeds from sales of securities available for sale |  |  | 36,021,000 |  |
Carrying Value | 36,021,000 |  | 36,021,000 |  |
Durbin Amendment Securities |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Proceeds from sales of securities available for sale | 446,971,000 |  |  |  |
Carrying Value | 446,880,000 |  | $ 446,880,000 |  |
Gross realized gain (loss), excluding other than temporary impairments | $ 91,000 |  |  |  |
Non-Heritage Securities [Member] |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Proceeds from sales of securities available for sale |  |  |  | 4,028,000 | 1,213,000
Carrying Value |  |  |  | 2,842,000 | 1,117,000
Gross realized gain (loss), excluding other than temporary impairments |  |  |  | 1,186,000 | 96,000
Heritage Securities |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Proceeds from sales of securities available for sale |  |  |  | $ 7,231,000 |
Carrying Value |  |  |  |  | $ 7,231,000

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