Acquisitions and Dispositions - Additional Information (Detail) shares in Millions								1 Months Ended	9 Months Ended	12 Months Ended
	Nov. 01, 2019 USD ($) branch shares	Oct. 25, 2019 USD ($) branch	Apr. 01, 2019 USD ($) branch shares	Oct. 01, 2018 USD ($) branch shares	Jun. 27, 2018 USD ($)	Aug. 01, 2017 USD ($)	Apr. 01, 2017 USD ($) Branches shares	Jun. 30, 2018 Securitization	Sep. 30, 2017 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 02, 2019 USD ($)
Business Acquisition [Line Items]
Loans outstanding principal balance										$ 532,200,000	$ 491,600,000
Goodwill										3,065,500,000	2,685,700,000	$ 2,411,400,000
Central Maine Branch Sale [Member]
Business Acquisition [Line Items]
Number of branches sold | branch		8
Deposits sold		$ 258,000,000
Loans sold		103,000,000
Assets under management sold		227,000,000
Gain on disposal		$ 7,600,000
United Financial [Member]
Business Acquisition [Line Items]
Business acquisition, date of acquisition agreement	Nov. 01, 2019
Business acquisition consideration transferred	$ 720,600,000
Business acquisition common stock shares (in shares) | shares	44.4
Number of branches acquired | branch	58
Merger-related expenses										$ 22,200,000
Net deferred tax liability	$ 16,700,000
Liabilities acquired	6,499,500,000
Goodwill	204,300,000
United Financial [Member] | Core Deposit Intangibles [Member]
Business Acquisition [Line Items]
Finite-lived intangible assets, useful life, (in years)										6 years
United Financial [Member] | Purchased Performing Loans [Member]
Business Acquisition [Line Items]
Loans	8,040,000,000.00
Loans outstanding principal balance	8,100,000,000
United Financial [Member] | Purchased Credit Impaired Loans [Member]
Business Acquisition [Line Items]
Loans	136,500,000
Total loans, net	193,100,000
BSB Bancorp [Member]
Business Acquisition [Line Items]
Business acquisition, date of acquisition agreement			Apr. 01, 2019
Business acquisition consideration transferred			$ 324,500,000
Business acquisition common stock shares (in shares) | shares			19.7
Number of branches acquired | branch			6
Merger-related expenses										$ 8,100,000	500,000
Net deferred tax liability			$ 3,800,000
Liabilities acquired			2,862,200,000
Goodwill			$ 144,900,000
BSB Bancorp [Member] | Core Deposit Intangibles [Member]
Business Acquisition [Line Items]
Finite-lived intangible assets, useful life, (in years)										10 years
BSB Bancorp [Member] | Purchased Credit Impaired Loans [Member]
Business Acquisition [Line Items]
Loans	$ 0
First Connecticut Bancorp, Inc. [Member]
Business Acquisition [Line Items]
Business acquisition, date of acquisition agreement				Oct. 01, 2018
Business acquisition consideration transferred				$ 486,400,000
Business acquisition common stock shares (in shares) | shares				28.4
Number of branches acquired | branch				25
Merger-related expenses										$ 16,800,000	8,800,000
Assets acquired				$ 3,450,000,000
Liabilities acquired				$ 2,960,000,000
Vend Lease [Member]
Business Acquisition [Line Items]
Business acquisition, date of acquisition agreement					Jun. 27, 2018
Merger-related expenses											$ 2,100,000
Loans					$ 68,800,000
Business acquisition cash consideration transferred					37,500,000
Goodwill					$ 23,900,000
LEAF Commercial Capital, Inc. [Member]
Business Acquisition [Line Items]
Business acquisition, date of acquisition agreement						Aug. 01, 2017
Merger-related expenses												3,700,000
Assets acquired						$ 957,700,000
Liabilities acquired						737,700,000
Business acquisition cash consideration transferred						$ 220,000,000.0
Repayment of borrowings									$ 460,000,000
Number of remaining securitization which can be repaid without penalty in 2018 | Securitization								1
Suffolk [Member]
Business Acquisition [Line Items]
Business acquisition, date of acquisition agreement							Apr. 01, 2017
Business acquisition consideration transferred							$ 484,800,000
Business acquisition common stock shares (in shares) | shares							26.6
Number of branches acquired | Branches							27
Merger-related expenses												$ 26,600,000
Assets acquired							$ 2,380,000,000
Liabilities acquired							$ 1,900,000,000
BSB Bancorp And United Financial [Member]
Business Acquisition [Line Items]
Interest income on loans acquired										125,000,000
VAR Technology Finance [Member]
Business Acquisition [Line Items]
Loans													$ 60,000,000.0
Payments for Merger Related Costs										$ 1,900,000