Fair Value Measurements - Recurring Basis (Details) - USD ($)	Jun. 30, 2020	Dec. 31, 2019
Assets:
Available-for-sale securities:	$ 1,002,970,000	$ 978,284,000
Equity securities	24,492,000	18,810,000
U.S. government and agencies
Assets:
Available-for-sale securities:	21,121,000	19,940,000
Government-sponsored entities
Assets:
Available-for-sale securities:	158,943,000	156,255,000
Municipal bonds
Assets:
Available-for-sale securities:	333,955,000	325,455,000
Mortgage-backed securities
Assets:
Available-for-sale securities:	488,951,000	476,634,000
Fair Value, Measurements, Recurring
Assets:
Available-for-sale securities:	1,002,970,000	978,284,000
Trading securities held in the “rabbi trust”	6,328,000	6,119,000
Liabilities:
Deferred compensation “rabbi trust”	6,320,000	6,112,000
Fair Value, Measurements, Recurring \| U.S. government and agencies
Assets:
Available-for-sale securities:	21,121,000	19,940,000
Fair Value, Measurements, Recurring \| Government-sponsored entities
Assets:
Available-for-sale securities:	158,943,000	156,255,000
Fair Value, Measurements, Recurring \| Municipal bonds
Assets:
Available-for-sale securities:	333,955,000	325,455,000
Fair Value, Measurements, Recurring \| Mortgage-backed securities
Assets:
Available-for-sale securities:	488,951,000	476,634,000
Fair Value, Measurements, Recurring \| Derivatives - interest rate customer swaps
Assets:
Derivatives	96,968,000	36,089,000
Liabilities:
Derivatives	97,760,000	36,580,000
Fair Value, Measurements, Recurring \| Interest rate swaps
Liabilities:
Derivatives	274,000
Fair Value, Measurements, Recurring \| Risk participation agreements
Assets:
Derivatives	21,000	10,000
Liabilities:
Derivatives	478,000	242,000
Fair Value, Measurements, Recurring \| Quoted prices in active markets for identical assets (Level 1)
Assets:
Available-for-sale securities:	0	0
Equity securities	24,492,000	18,810,000
Trading securities held in the “rabbi trust”	6,328,000	6,119,000
Liabilities:
Deferred compensation “rabbi trust”	6,320,000	6,112,000
Fair Value, Measurements, Recurring \| Quoted prices in active markets for identical assets (Level 1) \| U.S. government and agencies
Assets:
Available-for-sale securities:	0	0
Fair Value, Measurements, Recurring \| Quoted prices in active markets for identical assets (Level 1) \| Government-sponsored entities
Assets:
Available-for-sale securities:	0	0
Fair Value, Measurements, Recurring \| Quoted prices in active markets for identical assets (Level 1) \| Municipal bonds
Assets:
Available-for-sale securities:	0	0
Fair Value, Measurements, Recurring \| Quoted prices in active markets for identical assets (Level 1) \| Mortgage-backed securities
Assets:
Available-for-sale securities:	0	0
Fair Value, Measurements, Recurring \| Quoted prices in active markets for identical assets (Level 1) \| Derivatives - interest rate customer swaps
Assets:
Derivatives	0	0
Liabilities:
Derivatives	0	0
Fair Value, Measurements, Recurring \| Quoted prices in active markets for identical assets (Level 1) \| Interest rate swaps
Liabilities:
Derivatives	0
Fair Value, Measurements, Recurring \| Quoted prices in active markets for identical assets (Level 1) \| Risk participation agreements
Assets:
Derivatives	0	0
Liabilities:
Derivatives	0	0
Fair Value, Measurements, Recurring \| Significant other observable inputs (Level 2)
Assets:
Available-for-sale securities:	1,002,970,000	978,284,000
Equity securities	0	0
Trading securities held in the “rabbi trust”	0	0
Liabilities:
Deferred compensation “rabbi trust”	0	0
Fair Value, Measurements, Recurring \| Significant other observable inputs (Level 2) \| U.S. government and agencies
Assets:
Available-for-sale securities:	21,121,000	19,940,000
Fair Value, Measurements, Recurring \| Significant other observable inputs (Level 2) \| Government-sponsored entities
Assets:
Available-for-sale securities:	158,943,000	156,255,000
Fair Value, Measurements, Recurring \| Significant other observable inputs (Level 2) \| Municipal bonds
Assets:
Available-for-sale securities:	333,955,000	325,455,000
Fair Value, Measurements, Recurring \| Significant other observable inputs (Level 2) \| Mortgage-backed securities
Assets:
Available-for-sale securities:	488,951,000	476,634,000
Fair Value, Measurements, Recurring \| Significant other observable inputs (Level 2) \| Derivatives - interest rate customer swaps
Assets:
Derivatives	96,968,000	36,089,000
Liabilities:
Derivatives	97,760,000	36,580,000
Fair Value, Measurements, Recurring \| Significant other observable inputs (Level 2) \| Interest rate swaps
Liabilities:
Derivatives	274,000
Fair Value, Measurements, Recurring \| Significant other observable inputs (Level 2) \| Risk participation agreements
Assets:
Derivatives	21,000	10,000
Liabilities:
Derivatives	478,000	242,000
Fair Value, Measurements, Recurring \| Significant unobservable inputs (Level 3)
Assets:
Available-for-sale securities:	0	0
Equity securities	0	0
Trading securities held in the “rabbi trust”	0	0
Liabilities:
Deferred compensation “rabbi trust”	0	0
Fair Value, Measurements, Recurring \| Significant unobservable inputs (Level 3) \| U.S. government and agencies
Assets:
Available-for-sale securities:	0	0
Fair Value, Measurements, Recurring \| Significant unobservable inputs (Level 3) \| Government-sponsored entities
Assets:
Available-for-sale securities:	0	0
Fair Value, Measurements, Recurring \| Significant unobservable inputs (Level 3) \| Municipal bonds
Assets:
Available-for-sale securities:	0	0
Fair Value, Measurements, Recurring \| Significant unobservable inputs (Level 3) \| Mortgage-backed securities
Assets:
Available-for-sale securities:	0	0
Fair Value, Measurements, Recurring \| Significant unobservable inputs (Level 3) \| Derivatives - interest rate customer swaps
Assets:
Derivatives	0	0
Liabilities:
Derivatives	0	0
Fair Value, Measurements, Recurring \| Significant unobservable inputs (Level 3) \| Interest rate swaps
Liabilities:
Derivatives	0
Fair Value, Measurements, Recurring \| Significant unobservable inputs (Level 3) \| Risk participation agreements
Assets:
Derivatives	0	0
Liabilities:
Derivatives	$ 0	$ 0

Fair value of investment other than investment in equity security, investment in debt security and equity method investment. Includes, but is not limited to, investment in certain entities that calculate net asset value per share. Example includes, but is not limited to, investment in hedge fund, venture capital fund, private equity fund, and real estate partnership or fund.

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-Name Accounting Standards Codification
-Topic 820
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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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Fair value of financial and nonfinancial obligations.

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