Loans - Additional Information (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018		Jan. 31, 2020
Anticipated credit losses	$ (6,158,000)	$ 3,658,000	$ 1,868,000	$ 43,786,000	$ 1,084,000	$ 1,327,000	$ 282,000	$ 212,000	$ 43,154,000	[1]	$ 2,905,000	[1]	$ 1,176,000	[1]
Loan balance	5,847,484,000				2,874,688,000				5,847,484,000		2,874,688,000
Loans and Leases Receivable, Loans in Process	355,065,000				94,865,000				355,065,000		94,865,000
Purchase Credit Impaired [Member]
Loan balance	1,800,000								1,800,000
Loan Purchase [Member]
Purchase price of loans at acquisition	2,221,204,000								2,221,204,000
Fair value of purchase price of loans at acquisition	2,247,317,000								2,247,317,000						$ 79,100,000
Financing receivable credit discount															7,700,000
Financing receivable non credit discount															$ 4,100,000
Loan balance	65,801,000								65,801,000
Scenario Plan [Member]
Anticipated credit losses									5,300,000
Consumer Portfolio Segment
Loans and Leases Receivable, Loans in Process	784,000				981,000				784,000		981,000
Unfunded Loan Commitments [Member]
Unfunded loan commitments	1,300,000,000								1,300,000,000
COVID-19 [Member] | PPP [Member]
Cash paid for paycheck protection program									450,000,000
TDRs [Member]
Financing Receivable, Modifications, Recorded Investment	16,600,000				15,100,000				16,600,000		15,100,000
Specified Reserves, Provision for Troubled Debt Restructurings	883,000				388,000				883,000		388,000
Loans and Leases Receivable, Impaired, Commitment to Lend	303,000				$ 226,000				303,000		226,000
Loans receivable deferred	53,450,000								53,450,000
Commercial deferral expirations balance	46,038,000								46,038,000
Financing Receivables, Impaired, Troubled Debt Restructuring, Write-down									9,400,000
Financing Receivable, Allowance, Period Increase (decrease)									660,000		34,000		(110,000)
TDRs [Member] | Subsequently Defaulted [Member]
Financing Receivable, Allowance, Period Increase (decrease)									5,000		$ 4,000		$ 17,000
TDRs [Member] | COVID-19 [Member] | PPP [Member] | Other Loans [Member] | Commercial Loan Portfolio Segment [Member]
Loans outstanding	386,900,000								386,900,000
TDRs [Member] | COVID-19 [Member] | PPP [Member] | First Round of Originations [Member]
Loans outstanding	327,700,000								327,700,000
TDRs [Member] | COVID-19 [Member] | PPP [Member] | Second Round of Originations [Member]
Loans outstanding	139,300,000								139,300,000
TDRs [Member] | Minimum [Member]
Loans receivable deferred	$ 53,500,000								$ 53,500,000
TDRs [Member] | Minimum [Member] | COVID-19 [Member]
Loan modification payment term for either interest only deferral or principal and interest deferral									1 month
TDRs [Member] | Maximum [Member] | COVID-19 [Member]
Loan modification payment term for either interest only deferral or principal and interest deferral									9 months

[1] Beginning January 1, 2020, calculation is based on current expected loss methodology.  Prior to January 1, 2020, calculation was based on incurred loss methodology.