Loans - Narrative (Details)		3 Months Ended
	Jan. 01, 2020 USD ($)	Mar. 31, 2020 USD ($) loan	Mar. 31, 2019 USD ($)	Jun. 30, 2020 USD ($)	Apr. 30, 2020 USD ($)	Dec. 31, 2019 USD ($) loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Purchased with Credit Deterioration, Discount (Premium)		$ 24,500,000
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					$ 392,100,000
Financing Receivable, Amount Eligible for Deferral of Both Principal and Interest					190,800,000
Loans and Leases Receivable, Deferred Income					$ 2,200,000
Federal Home Loan Bank, Advances, General Debt Obligations, Maximum Amount Available		2,700,000,000
Financing receivable, allowance for credit loss	$ 43,288,000
Financing Receivable, Paycheck Protection Program				$ 856,500,000
Unadvanced Commitment on Impaired Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Estimated losses attributable to unadvanced commitments on impaired loans		3,700,000				400,000
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)	21,695,000	$ 1,500,000
Financing Receivable, Allowance for Credit Loss, Threshold of Loans Written Off	100,000
Post-Modification Outstanding Recorded Balance		$ 3,886,000
Financing Receivable, Modifications, Number of Contracts | loan		3
Restructuring Reserve		$ 3,810,000				79,000
Financing Receivable, before Allowance for Credit Loss		5,459,967,000				5,316,953,000
Financing receivable, allowance for credit loss	69,995,000	92,187,000				43,288,000
Non-Covered Loans | Consumer and 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	566,000
Restructuring Reserve		0				0
Financing Receivable, before Allowance for Credit Loss		126,212,000				134,941,000
Financing receivable, allowance for credit loss	1,050,000	1,547,000				1,134,000
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		$ 2,200,000