Financial Instruments - Summary of Contractual or Notional Amounts of Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Commercial and Industrial [Member]
Derivative Instruments And Hedging Activities [Line Items]
Loan origination commitments and unadvanced lines of credit:	$ 5,549.9	$ 4,937.1
Home Equity and Other Consumer [Member]
Derivative Instruments And Hedging Activities [Line Items]
Loan origination commitments and unadvanced lines of credit:	3,401.9	2,646.6
Commercial Real Estate [Member]
Derivative Instruments And Hedging Activities [Line Items]
Loan origination commitments and unadvanced lines of credit:	1,614.9	1,095.4
Equipment Financing [Member]
Derivative Instruments And Hedging Activities [Line Items]
Loan origination commitments and unadvanced lines of credit:	471.0	459.7
Residential Mortgage [Member]
Derivative Instruments And Hedging Activities [Line Items]
Loan origination commitments and unadvanced lines of credit:	54.7	78.7
Forward Commitments to Sell Residential Mortgage Loans [Member]
Derivative Instruments And Hedging Activities [Line Items]
Derivative Financial Instruments	23.3	9.5
Interest Rate-Lock Commitments on Residential Mortgage Loans [Member]
Derivative Instruments And Hedging Activities [Line Items]
Derivative Financial Instruments	33.6	13.6
Stand-By Letters of Credit [Member]
Derivative Instruments And Hedging Activities [Line Items]
Letters of credit:	185.2	139.4
Commercial Letters of Credit [Member]
Derivative Instruments And Hedging Activities [Line Items]
Letters of credit:	4.4	5.0
Interest Rate Swaps [Member] | Subordinated Notes [Member]
Derivative Instruments And Hedging Activities [Line Items]
Derivative Financial Instruments	585.0	585.0
Interest Rate Swaps [Member] | Customer [Member]
Derivative Instruments And Hedging Activities [Line Items]
Derivative Financial Instruments	8,847.7	7,455.9
Interest Rate Swaps [Member] | Counterparty [Member]
Derivative Instruments And Hedging Activities [Line Items]
Derivative Financial Instruments	8,851.8	7,161.3
Interest Rate Caps [Member] | Customer [Member]
Derivative Instruments And Hedging Activities [Line Items]
Derivative Financial Instruments	246.0	329.1
Interest Rate Caps [Member] | Counterparty [Member]
Derivative Instruments And Hedging Activities [Line Items]
Derivative Financial Instruments	246.0	329.1
Risk Participation Agreements [Member]
Derivative Instruments And Hedging Activities [Line Items]
Derivative Financial Instruments	882.8	576.5
Foreign Exchange Contracts [Member]
Derivative Instruments And Hedging Activities [Line Items]
Derivative Financial Instruments	$ 180.4	$ 145.2