Fair Value Measurements and Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Transfers between Levels [Abstract]
Transfers from Level 1 to Level 2	$ 0	$ 0
Transfers from Level 2 to Level 1	4,000	0
Transfers into Level 3		0
Transfers out of Level 3		0
AFS securities [Abstract]
AFS securities	1,101,074
Total AFS securities	1,101,074	1,255,925
Equity securities	24,722	0
Trading securities	0	11,467
Liabilities [Abstract]
Collateral dependent loans	25	100
Collateral dependent loans, specific reserves	$ 25	100
Minimum [Member]
Liabilities [Abstract]
Liquidation expense ratio on impaired collateral	10.00%
Maximum [Member]
Liabilities [Abstract]
Liquidation expense ratio on impaired collateral	35.00%
Federal Agency [Member]
AFS securities [Abstract]
AFS securities	$ 83,262	108,899
State & Municipal [Member]
AFS securities [Abstract]
AFS securities	36,467	41,956
Recurring Basis [Member]
AFS securities [Abstract]
AFS securities	1,101,074
Other securities		14,149
Total AFS securities		1,255,925
Equity securities	24,722
Trading securities		11,467
Derivatives	23,682	3,732
Total	1,149,478	1,271,124
Liabilities [Abstract]
Derivatives	20,054	324
Total	20,054	324
Recurring Basis [Member] \| Level 1 [Member]
AFS securities [Abstract]
AFS securities	0
Other securities		5,845
Total AFS securities		5,845
Equity securities	20,722
Trading securities		11,467
Derivatives	0	0
Total	20,722	17,312
Liabilities [Abstract]
Derivatives	0	0
Total	0	0
Recurring Basis [Member] \| Level 2 [Member]
AFS securities [Abstract]
AFS securities	1,101,074
Other securities		8,304
Total AFS securities		1,250,080
Equity securities	4,000
Trading securities		0
Derivatives	23,682	3,732
Total	1,128,756	1,253,812
Liabilities [Abstract]
Derivatives	20,054	324
Total	20,054	324
Recurring Basis [Member] \| Level 3 [Member]
AFS securities [Abstract]
AFS securities	0
Other securities		0
Total AFS securities		0
Equity securities	0
Trading securities		0
Derivatives	0	0
Total	0	0
Liabilities [Abstract]
Derivatives	0	0
Total	0	0
Recurring Basis [Member] \| Federal Agency [Member]
AFS securities [Abstract]
AFS securities	83,262	108,899
Recurring Basis [Member] \| Federal Agency [Member] \| Level 1 [Member]
AFS securities [Abstract]
AFS securities	0	0
Recurring Basis [Member] \| Federal Agency [Member] \| Level 2 [Member]
AFS securities [Abstract]
AFS securities	83,262	108,899
Recurring Basis [Member] \| Federal Agency [Member] \| Level 3 [Member]
AFS securities [Abstract]
AFS securities	0	0
Recurring Basis [Member] \| State & Municipal [Member]
AFS securities [Abstract]
AFS securities	36,467	41,956
Recurring Basis [Member] \| State & Municipal [Member] \| Level 1 [Member]
AFS securities [Abstract]
AFS securities	0	0
Recurring Basis [Member] \| State & Municipal [Member] \| Level 2 [Member]
AFS securities [Abstract]
AFS securities	36,467	41,956
Recurring Basis [Member] \| State & Municipal [Member] \| Level 3 [Member]
AFS securities [Abstract]
AFS securities	0	0
Recurring Basis [Member] \| Mortgage-Backed [Member]
AFS securities [Abstract]
AFS securities	492,408	554,927
Recurring Basis [Member] \| Mortgage-Backed [Member] \| Level 1 [Member]
AFS securities [Abstract]
AFS securities	0	0
Recurring Basis [Member] \| Mortgage-Backed [Member] \| Level 2 [Member]
AFS securities [Abstract]
AFS securities	492,408	554,927
Recurring Basis [Member] \| Mortgage-Backed [Member] \| Level 3 [Member]
AFS securities [Abstract]
AFS securities	0	0
Recurring Basis [Member] \| Collateralized Mortgage Obligations [Member]
AFS securities [Abstract]
AFS securities	488,937	535,994
Recurring Basis [Member] \| Collateralized Mortgage Obligations [Member] \| Level 1 [Member]
AFS securities [Abstract]
AFS securities	0	0
Recurring Basis [Member] \| Collateralized Mortgage Obligations [Member] \| Level 2 [Member]
AFS securities [Abstract]
AFS securities	488,937	535,994
Recurring Basis [Member] \| Collateralized Mortgage Obligations [Member] \| Level 3 [Member]
AFS securities [Abstract]
AFS securities	$ 0	$ 0

The percentage of liquidation expenses estimated with liquidating impaired collateral.

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Amount of loans considered to be collateral dependent.

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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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).

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

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Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

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Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

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Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

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Fair value of financial and nonfinancial obligations.

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Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading) and investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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