Goodwill and Other Acquisition-Related Intangible Assets - Additional Information (Detail)	3 Months Ended	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Oct. 25, 2019 branch
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill expected to be tax deductible	$ 134,400,000	$ 134,400,000	$ 90,300,000
Weighted-average amortization period		12 years
Amortization of other acquisition-related intangible assets		$ 32,500,000	21,800,000	$ 30,000,000.0
Impairment losses relating to goodwill or other acquisition-related intangible assets		0	$ 0	$ 0
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
2020	40,900,000	40,900,000
2021	36,100,000	36,100,000
2022	31,700,000	31,700,000
2023	24,000,000.0	24,000,000.0
2024	20,300,000	$ 20,300,000
Central Maine Branch Sale [Member]
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
Number of branches sold | branch					8
Mutual Fund Management Contract [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Intangible asset write-down	16,500,000
Trade Name [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Intangible asset write-down	1,900,000
Customer Relationships [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Intangible asset write-down	$ 1,000,000.0