Loans - Narrative (Details)		3 Months Ended		9 Months Ended
	Jan. 01, 2020 USD ($)	Sep. 30, 2020 USD ($) loan	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) loan	Sep. 30, 2019 USD ($) loan	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($) loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Purchased with Credit Deterioration, Discount (Premium)				$ 19,800,000
Post-Modification Outstanding Recorded Balance		$ 3,933,000
Financing Receivable, Modifications, Number of Contracts | loan		3
Number of loans over 90 days past due and still accruing interest | loan							0
Unadvanced commitments on impaired loans							$ 225,000
Financing Receivable, Amount Eligible for Deferred Payment		$ 690,800,000		690,800,000
Financing Receivable, Amount Eligible for Deferral of Both Principal and Interest		355,800,000		355,800,000
Financing Receivable, Amount Eligible for Deferral of Principal Only		335,000,000.0		335,000,000.0
Loans and Leases Receivable, Deferred Income		4,500,000		4,500,000
Federal Home Loan Bank, Advances, General Debt Obligations, Maximum Amount Available		2,400,000,000		2,400,000,000
Financing receivable, allowance for credit loss	$ 43,288,000
Financing Receivable, Paycheck Protection Program		838,600,000		838,600,000
Financing Receivable, Paycheck Protection Program, Net of Deferred Fees		819,100,000		819,100,000
Financing Receivable, Paycheck Protection Program, Unearned Fees		19,500,000		19,500,000
Financing Receivable, Amount Still in Deferral Status		139,400,000		139,400,000
Unadvanced Commitment on Impaired Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Estimated losses attributable to unadvanced commitments on impaired loans		5,192,000		5,192,000			430,000
Non-Covered Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Loss, Period Increase (Decrease)	28,387,000
Provision for credit losses	(1,680,000)	14,000,000.0	$ 1,800,000	54,100,000	$ 5,000,000.0
Financing Receivable, Allowance for Credit Loss, Threshold of Loans Written Off	100,000
Post-Modification Outstanding Recorded Balance				$ 7,819,000	$ 520,000
Financing Receivable, Modifications, Number of Contracts | loan				6	3
Restructuring Reserve		3,635,000		$ 3,635,000			79,000
Financing Receivable, before Allowance for Credit Loss		6,147,551,000		6,147,551,000			5,316,953,000
Financing receivable, allowance for credit loss	69,995,000	123,270,000		123,270,000		$ 110,270,000	43,288,000
Non-Covered Loans | Enterprise Value Lending Portfolio Niche Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, allowance for credit loss		20,100,000		20,100,000
Non-Covered Loans | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Loss, Period Increase (Decrease)	(84,000)
Provision for credit losses	0
Restructuring Reserve		0		0			0
Financing Receivable, before Allowance for Credit Loss		151,230,000		151,230,000			134,941,000
Financing receivable, allowance for credit loss	1,050,000	2,008,000		2,008,000		$ 1,729,000	1,134,000
Non-Covered Loans | Agriculture Portfolio Niche Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, allowance for credit loss		1,800,000		1,800,000
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Post-Modification Outstanding Recorded Balance		$ 3,716,000
Financing Receivable, Modifications, Number of Contracts | loan		2
Financing Receivable, Troubled Debt Restructuring, Subsequent Default			100,000		$ 400,000
Financing Receivable, Troubled Debt Restructuring, Subsequent Default			$ 100,000		$ 400,000
Financial Asset Acquired with Credit Deterioration [Member] | Non-Covered Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, before Allowance for Credit Loss							$ 90,300,000
Cumulative Effect, Period Of Adoption, Adjustment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, allowance for credit loss	$ 28,387,000
Cumulative Effect, Period Of Adoption, Adjustment | Non-Covered Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, allowance for credit loss		$ 1,200,000		$ 1,200,000