FAIR VALUE MEASUREMENTS - Fair Value Recurring Basis (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Assets:
Available-for-sale, fair value	$ 1,243,693,000	$ 978,284,000
Equity securities at fair value	41,452,000	18,810,000
U.S. government and agencies
Assets:
Available-for-sale, fair value	20,984,000	19,940,000
Government-sponsored entities
Assets:
Available-for-sale, fair value	147,786,000	156,255,000
Municipal bonds
Assets:
Available-for-sale, fair value	346,588,000	325,455,000
Mortgage-backed securities
Assets:
Available-for-sale, fair value	728,335,000	476,634,000
Fair Value, Measurements, Recurring
Assets:
Available-for-sale, fair value	1,243,693,000	978,284,000
Equity securities at fair value		18,810,000
Trading securities held in the rabbi trust	7,204,000	6,119,000
Liabilities:
Deferred compensation rabbi trust	7,204,000	6,112,000
Fair Value, Measurements, Recurring \| U.S. government and agencies
Assets:
Available-for-sale, fair value	20,984,000	19,940,000
Fair Value, Measurements, Recurring \| Government-sponsored entities
Assets:
Available-for-sale, fair value	147,786,000	156,255,000
Fair Value, Measurements, Recurring \| Municipal bonds
Assets:
Available-for-sale, fair value	346,588,000	325,455,000
Fair Value, Measurements, Recurring \| Mortgage-backed securities
Assets:
Available-for-sale, fair value	728,335,000	476,634,000
Fair Value, Measurements, Recurring \| Derivatives - interest rate customer swaps
Assets:
Derivative Assets	83,255,000	36,089,000
Liabilities:
Derivative liabilities	84,590,000	36,580,000
Fair Value, Measurements, Recurring \| Derivatives - interest rate swaps
Liabilities:
Derivative liabilities	228,000
Fair Value, Measurements, Recurring \| Risk participation agreements
Assets:
Derivative Assets	49,000	10,000
Liabilities:
Derivative liabilities	375,000	242,000
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 1
Assets:
Available-for-sale, fair value	0	0
Equity securities at fair value	41,452,000	18,810,000
Trading securities held in the rabbi trust	7,204,000	6,119,000
Liabilities:
Deferred compensation rabbi trust	7,204,000	6,112,000
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 1 \| U.S. government and agencies
Assets:
Available-for-sale, fair value	0	0
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 1 \| Government-sponsored entities
Assets:
Available-for-sale, fair value	0	0
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 1 \| Municipal bonds
Assets:
Available-for-sale, fair value	0	0
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 1 \| Mortgage-backed securities
Assets:
Available-for-sale, fair value	0	0
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 1 \| Derivatives - interest rate customer swaps
Assets:
Derivative Assets	0	0
Liabilities:
Derivative liabilities	0	0
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 1 \| Derivatives - interest rate swaps
Liabilities:
Derivative liabilities	0
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 1 \| Risk participation agreements
Assets:
Derivative Assets	0	0
Liabilities:
Derivative liabilities	0	0
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2
Assets:
Available-for-sale, fair value	1,243,693,000	978,284,000
Equity securities at fair value	0	0
Trading securities held in the rabbi trust	0	0
Liabilities:
Deferred compensation rabbi trust	0	0
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2 \| U.S. government and agencies
Assets:
Available-for-sale, fair value	20,984,000	19,940,000
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2 \| Government-sponsored entities
Assets:
Available-for-sale, fair value	147,786,000	156,255,000
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2 \| Municipal bonds
Assets:
Available-for-sale, fair value	346,588,000	325,455,000
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2 \| Mortgage-backed securities
Assets:
Available-for-sale, fair value	728,335,000	476,634,000
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2 \| Derivatives - interest rate customer swaps
Assets:
Derivative Assets	83,255,000	36,089,000
Liabilities:
Derivative liabilities	84,590,000	36,580,000
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2 \| Derivatives - interest rate swaps
Liabilities:
Derivative liabilities	228,000
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2 \| Risk participation agreements
Assets:
Derivative Assets	49,000	10,000
Liabilities:
Derivative liabilities	375,000	242,000
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 3
Assets:
Available-for-sale, fair value	0	0
Equity securities at fair value	0	0
Trading securities held in the rabbi trust	0	0
Liabilities:
Deferred compensation rabbi trust	0	0
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 3 \| U.S. government and agencies
Assets:
Available-for-sale, fair value	0	0
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 3 \| Government-sponsored entities
Assets:
Available-for-sale, fair value	0	0
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 3 \| Municipal bonds
Assets:
Available-for-sale, fair value	0	0
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 3 \| Mortgage-backed securities
Assets:
Available-for-sale, fair value	0	0
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 3 \| Derivatives - interest rate customer swaps
Assets:
Derivative Assets	0	0
Liabilities:
Derivative liabilities	0	0
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 3 \| Derivatives - interest rate swaps
Liabilities:
Derivative liabilities	0
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 3 \| Risk participation agreements
Assets:
Derivative Assets	0	0
Liabilities:
Derivative liabilities	$ 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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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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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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