Loans - Additional Information (Details) - USD ($)		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jan. 31, 2020	Dec. 31, 2019
Cash paid for paycheck protection program				$ 443,300,000
Anticipated credit losses	[1]	$ 3,658,000	$ 1,327,000	49,312,000	$ 1,821,000
Loan balance		5,381,631,000		5,381,631,000			$ 2,715,078,000
Loans and Leases Receivable, Loans in Process		300,174,000		300,174,000			126,108,000
Consumer Portfolio Segment [Member]
Loans and Leases Receivable, Loans in Process		6,000,000.0		6,000,000.0			981,000
Purchase Credit Impaired [Member]
Loan balance		1,800,000		1,800,000
Loan Purchase [Member]
Purchase price of loans at acquisition		2,340,701,000		2,340,701,000
Fair value of purchase price of loans at acquisition		2,314,588,000		2,314,588,000		$ 79,100,000
Financing receivable credit discount						7,700,000
Financing receivable non credit discount						$ 4,100,000
Loan balance		75,063,000		75,063,000
Scenario Plan [Member]
Anticipated credit losses				6,000,000.0
Unfunded Loan Commitments [Member]
Unfunded loan commitments		1,400,000,000		1,400,000,000
Subsequently Defaulted [Member]
Financing Receivable, Allowance for Credit Losses, Period Increase (decrease)		16,000		61,000
TDRs [Member]
Financing Receivable, Modifications, Recorded Investment		18,800,000		18,800,000			15,100,000
Specified Reserves, Provision for Troubled Debt Restructurings		1,067,000		1,067,000			388,000
Loans and Leases Receivable, Impaired, Commitment to Lend		389,000		389,000			$ 226,000
Loans receivable deferred		482,742,000		482,742,000
Commercial deferral expirations balance		434,555,000		434,555,000
Financing Receivables, Impaired, Troubled Debt Restructuring, Write-down				10,400,000
Financing Receivable, Allowance for Credit Losses, Period Increase (decrease)		758,000	11,000	790,000	32,000
TDRs [Member] | Subsequently Defaulted [Member]
Financing Receivable, Allowance for Credit Losses, Period Increase (decrease)			$ 0		$ 4,000
TDRs [Member] | Minimum [Member]
Loans receivable deferred		$ 482,700,000		$ 482,700,000
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.