Allowance for Loan Losses - Allowance by Impairment Analysis Method (Details) - USD ($)	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Line Items]
Individually Evaluated for Impairment Recorded investment (loan balance)	$ 25,339,000	$ 25,142,000
Individually Evaluated for Impairment Allowance for loan losses	734,000	400,000
Collectively Evaluated for Impairment Recorded investment (loan balance)	7,190,986,000	7,079,167,000
Collectively Evaluated for Impairment Allowance for loan losses	73,276,000	80,838,000
Net loans	7,216,325,000	7,104,309,000
Allowance for loan losses	74,010,000	81,238,000	$ 68,211,000	$ 71,982,000
Commercial and industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
Individually Evaluated for Impairment Recorded investment (loan balance)	4,416,000	4,315,000
Individually Evaluated for Impairment Allowance for loan losses	541,000	279,000
Collectively Evaluated for Impairment Recorded investment (loan balance)	663,801,000	554,028,000
Collectively Evaluated for Impairment Allowance for loan losses	10,203,000	8,706,000
Net loans	668,217,000	558,343,000
Allowance for loan losses	10,744,000	8,985,000	10,255,000	10,048,000
Paycheck Protection Program
Financing Receivable, Allowance for Credit Loss [Line Items]
Individually Evaluated for Impairment Recorded investment (loan balance)	0	0
Individually Evaluated for Impairment Allowance for loan losses	0	0
Collectively Evaluated for Impairment Recorded investment (loan balance)	351,170,000	312,356,000
Collectively Evaluated for Impairment Allowance for loan losses	179,000	159,000
Net loans	351,170,000	312,356,000
Allowance for loan losses	179,000	159,000
Commercial tax-exempt
Financing Receivable, Allowance for Credit Loss [Line Items]
Individually Evaluated for Impairment Recorded investment (loan balance)	0	0
Individually Evaluated for Impairment Allowance for loan losses	0	0
Collectively Evaluated for Impairment Recorded investment (loan balance)	488,507,000	442,159,000
Collectively Evaluated for Impairment Allowance for loan losses	2,585,000	2,550,000
Net loans	488,507,000	442,159,000
Allowance for loan losses	2,585,000	2,550,000	1,927,000	6,016,000
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Individually Evaluated for Impairment Recorded investment (loan balance)	5,261,000	5,262,000
Individually Evaluated for Impairment Allowance for loan losses	125,000	50,000
Collectively Evaluated for Impairment Recorded investment (loan balance)	2,692,416,000	2,752,113,000
Collectively Evaluated for Impairment Allowance for loan losses	45,632,000	51,111,000
Net loans	2,697,677,000	2,757,375,000
Allowance for loan losses	45,757,000	51,161,000	36,580,000	40,765,000
Construction and land
Financing Receivable, Allowance for Credit Loss [Line Items]
Individually Evaluated for Impairment Recorded investment (loan balance)	0	0
Individually Evaluated for Impairment Allowance for loan losses	0	0
Collectively Evaluated for Impairment Recorded investment (loan balance)	181,482,000	159,204,000
Collectively Evaluated for Impairment Allowance for loan losses	3,253,000	4,041,000
Net loans	181,482,000	159,204,000
Allowance for loan losses	3,253,000	4,041,000	5,709,000	5,119,000
Residential
Financing Receivable, Allowance for Credit Loss [Line Items]
Individually Evaluated for Impairment Recorded investment (loan balance)	15,051,000	14,942,000
Individually Evaluated for Impairment Allowance for loan losses	51,000	54,000
Collectively Evaluated for Impairment Recorded investment (loan balance)	2,617,503,000	2,662,522,000
Collectively Evaluated for Impairment Allowance for loan losses	10,127,000	12,810,000
Net loans	2,632,554,000	2,677,464,000
Allowance for loan losses	10,178,000	12,864,000	11,779,000	8,857,000
Home equity
Financing Receivable, Allowance for Credit Loss [Line Items]
Individually Evaluated for Impairment Recorded investment (loan balance)	611,000	623,000
Individually Evaluated for Impairment Allowance for loan losses	17,000	17,000
Collectively Evaluated for Impairment Recorded investment (loan balance)	71,141,000	76,741,000
Collectively Evaluated for Impairment Allowance for loan losses	212,000	276,000
Net loans	71,752,000	77,364,000
Allowance for loan losses	229,000	293,000	303,000	778,000
Consumer and other
Financing Receivable, Allowance for Credit Loss [Line Items]
Individually Evaluated for Impairment Recorded investment (loan balance)	0	0
Individually Evaluated for Impairment Allowance for loan losses	0	0
Collectively Evaluated for Impairment Recorded investment (loan balance)	124,966,000	120,044,000
Collectively Evaluated for Impairment Allowance for loan losses	1,085,000	1,185,000
Net loans	124,966,000	120,044,000
Allowance for loan losses	1,085,000	1,185,000	$ 1,658,000	$ 399,000
Financial Asset Acquired with Credit Deterioration
Financing Receivable, Allowance for Credit Loss [Line Items]
Net loans	$ 0	$ 0

Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

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