Loans - Additional Information (Detail)		12 Months Ended
	Apr. 16, 2010 USD ($)	Dec. 31, 2019 USD ($) Segments Rating	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of segments (in segments) | Segments		2
Net deferred loan costs		$ 87,500,000	$ 94,600,000
Construction loans		1,100,000,000	651,200,000
Interest-only residential mortgage loans		1,100,000,000	1,200,000,000
Net gains on sales of residential mortgage loans		1,900,000	1,200,000	$ 3,200,000
Loans held-for-sale		511,300,000	19,500,000
Acquired loans sold, outstanding balance		10,100,000	10,000,000.0	7,900,000
Acquired loans sold, carrying amount		7,100,000	4,400,000	5,000,000.0
Net gains (losses) on sales of acquired loan		400,000	1,800,000	2,400,000
Recorded investments, non-performing loans		157,000,000.0	167,700,000	148,700,000
Interest income that would have recognized		18,500,000	18,100,000	16,200,000
Allowance for loan losses		246,600,000	240,400,000	234,400,000	$ 229,300,000
Interest Income Recognized		$ 5,700,000	6,400,000	5,600,000
Interest rate reduction		2.00%
Collateral values/LTV ratios, minimum		70.00%
Borrower credit scores, minimum | Rating		680
Principal and interest payments receivable		$ 7,950,000,000
Expected cash flows		7,350,000,000
Loan portfolio fair value		5,620,000,000
Aggregate loan nonaccretable difference		603,100,000
Portfolio accretable yield		137,500,000	189,700,000	219,700,000	255,400,000
Portfolio outstanding balance		532,200,000	491,600,000
Portfolio carrying value		396,000,000.0	399,900,000
FDIC loss-share receivable		$ 100,000	$ 200,000
Connecticut [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total loan portfolio representing loans to customers		25.00%
Massachusetts [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total loan portfolio representing loans to customers		22.00%
New York State [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total loan portfolio representing loans to customers		18.00%
New England [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total loan portfolio representing loans to customers		58.00%	56.00%
Residential Mortgage Construction Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Construction loans		$ 40,500,000	$ 57,300,000
Construction loan not yet advanced		14,600,000	21,500,000
Commercial Real Estate Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Early non-performing loans		8,600,000	20,300,000
Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held-for-sale		157,900,000
Early non-performing loans		18,800,000	15,800,000
Equipment Financing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Early non-performing loans		$ 26,200,000	$ 28,400,000
Equipment Financing [Member] | Outside New England [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total loan portfolio representing loans to customers		95.00%	95.00%
Equipment Financing [Member] | Texas California and New York [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total loan portfolio representing loans to customers		30.00%
Equipment Financing [Member] | Outside Texas California and New York [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total loan portfolio representing loans to customers		7.00%
Residential Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held-for-sale		$ 19,700,000	$ 19,500,000
Commercial Construction Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Construction loan not yet advanced		798,300,000	588,200,000
Other Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held-for-sale		333,700,000
Acquired Loans [Member] | Subsequent to Acquisition [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded investments, non-performing loans		13,200,000	6,000,000.0	4,700,000
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded investments, non-performing loans		108,100,000	113,500,000	100,600,000
Allowance for loan losses		217,900,000	209,500,000	204,500,000	204,900,000
Interest Income Recognized		$ 3,000,000.0	4,000,000.0	3,500,000
Duration of extension for payment deferral on TDRs, years		2 years
Commercial [Member] | Commercial Real Estate Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded investments, non-performing loans		$ 29,800,000	33,500,000	23,700,000
Interest Income Recognized		1,100,000	1,100,000	1,200,000
Commercial [Member] | Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded investments, non-performing loans		32,100,000	38,000,000.0	32,600,000
Interest Income Recognized		1,600,000	2,800,000	1,900,000
Commercial [Member] | Equipment Financing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded investments, non-performing loans		46,200,000	42,000,000.0	44,300,000
Interest Income Recognized		300,000	100,000	400,000
Retail Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded investments, non-performing loans		48,900,000	54,200,000	48,100,000
Allowance for loan losses		28,700,000	30,900,000	29,900,000	$ 24,400,000
Interest Income Recognized		$ 2,700,000	2,400,000	2,100,000
Duration of extension for payment deferral on TDRs, years		5 years
Retail Loans [Member] | Other Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded investments, non-performing loans		$ 0	0	0
Interest Income Recognized		0	0	0
Butler Bank [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
FDIC reimbursement for future losses in percentage	80.00%
FDIC loss share reimbursement value maximum	$ 34,000,000.0
Bank reimbursement to FDIC for future recoveries of losses reimbursed to bank by FDIC	80.00%
At Acquisition [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Portfolio accretable yield		1,730,000,000
Acquired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded investments, non-performing loans		53,500,000	44,100,000	25,100,000
Acquired Loans [Member] | Purchased Performing Loans [Member] | Subsequent to Acquisition [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded investments, non-performing loans		13,600,000	6,000,000.0	4,700,000
Non-Performing Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest income on loans acquired		2,300,000	2,900,000	1,700,000
Troubled Debt Restructurings [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded investment in originated loans classified as TDRs		177,000,000.0	179,400,000
Allowance for loan losses		4,300,000	4,500,000
Interest Income Recognized		5,500,000	6,100,000	$ 4,700,000
Foreclosure or Bankruptcy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Early non-performing loans		$ 20,800,000	$ 19,100,000