Loans - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2020 USD ($) Rating Segment	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of segments | Segment	2
Net deferred loan costs	$ 73,500,000		$ 87,500,000
Nonaccrual interest income	500,000
Loans past due and accruing	0		0
Allowance for loan losses	$ 341,700,000	$ 240,900,000	246,600,000	$ 240,400,000
Temporary reduction of interest rate for TDRs, basis points	2.00%
Collateral values/LTV ratios, minimum	70.00%
Borrower credit scores, minimum | Rating	680
Repossessed assets	$ 4,600,000		4,200,000
Loans held-for-sale	19,200,000		511,300,000
Gain (Loss) on Sale of Loans and Leases	15,500,000	400,000
Nonrecurring
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held-for-sale	19,200,000
Other Assets
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable accrued interest	111,900,000
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Early non-performing loans	24,200,000		27,600,000
Real estate owned	7,300,000		7,300,000
Commercial real estate | Nonrecurring
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held-for-sale			157,900,000
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Early non-performing loans	33,600,000		23,000,000.0
Equipment financing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Early non-performing loans	19,400,000		26,200,000
MWABL
Accounts, Notes, Loans and Financing Receivable [Line Items]
Early non-performing loans	1,100,000
Residential mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Real estate owned	9,500,000		11,900,000
Residential mortgage | Nonrecurring
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held-for-sale	19,200,000		19,700,000
Other consumer | Nonrecurring
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held-for-sale			333,700,000
Other consumer | Nonrecurring | United Financial Bancorp, Inc
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held-for-sale			333,700,000
Commercial Loan | Nonrecurring | United Financial Bancorp, Inc
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held-for-sale			157,900,000
Consumer And Commercial Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gain (Loss) on Sale of Loans and Leases	16,900,000
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	211,800,000	209,000,000.0	217,900,000	209,500,000
Commercial | Collateral Pledged
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	28,100,000
Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	129,900,000	31,900,000	28,700,000	$ 30,900,000
Retail | Collateral Pledged
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 29,200,000
Maximum | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Duration of extension for payment deferral on TDRs, years	2 years
Maximum | Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Duration of extension for payment deferral on TDRs, years	5 years
Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan agreement	6 months
Troubled Debt Restructurings
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded investment in originated loans classified as TDRs	$ 178,900,000		177,000,000.0
Allowance for loan losses	13,100,000		4,300,000
Interest income recognized	1,300,000	$ 1,600,000
Foreclosure or Bankruptcy
Accounts, Notes, Loans and Financing Receivable [Line Items]
Early non-performing loans	$ 38,000,000.0		$ 36,800,000