Financial Instruments - Impact of Derivatives on Pre-Tax Income and Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	$ 24.2	$ 16.3	$ 19.2
Amount of Pre-Tax Gain (Loss) Recognized in AOCL	1.9
Amount of Pre-Tax Gain (Loss) Recognized in AOCL		(1.7)	(1.0)
Derivatives Not Designated as Hedging Instruments [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	25.0	14.7	12.4
Amount of Pre-Tax Gain (Loss) Recognized in AOCL	0.0
Amount of Pre-Tax Gain (Loss) Recognized in AOCL		0.0	0.0
Derivatives Not Designated as Hedging Instruments [Member] | Forward Commitments to Sell Residential Mortgage Loans [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	0.3	(0.1)	(0.2)
Amount of Pre-Tax Gain (Loss) Recognized in AOCL	0.0
Amount of Pre-Tax Gain (Loss) Recognized in AOCL		0.0	0.0
Derivatives Not Designated as Hedging Instruments [Member] | Interest Rate-Lock Commitments on Residential Mortgage Loans [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	(0.4)	0.1	0.3
Amount of Pre-Tax Gain (Loss) Recognized in AOCL	0.0
Amount of Pre-Tax Gain (Loss) Recognized in AOCL		0.0	0.0
Derivatives Not Designated as Hedging Instruments [Member] | Interest Rate Swaps [Member] | Commercial Customers [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	374.9	(3.6)	5.3
Amount of Pre-Tax Gain (Loss) Recognized in AOCL	0.0
Amount of Pre-Tax Gain (Loss) Recognized in AOCL		0.0	0.0
Derivatives Not Designated as Hedging Instruments [Member] | Interest Rate Swaps [Member] | Institutional Counterparties [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	(350.3)	17.1	6.1
Amount of Pre-Tax Gain (Loss) Recognized in AOCL	0.0
Amount of Pre-Tax Gain (Loss) Recognized in AOCL		0.0	0.0
Derivatives Not Designated as Hedging Instruments [Member] | Interest Rate Caps [Member] | Commercial Customers [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	1.4	1.1	0.7
Amount of Pre-Tax Gain (Loss) Recognized in AOCL	0.0
Amount of Pre-Tax Gain (Loss) Recognized in AOCL		0.0	0.0
Derivatives Not Designated as Hedging Instruments [Member] | Interest Rate Caps [Member] | Institutional Counterparties [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	(1.5)	(1.0)	(0.3)
Amount of Pre-Tax Gain (Loss) Recognized in AOCL	0.0
Amount of Pre-Tax Gain (Loss) Recognized in AOCL		0.0	0.0
Derivatives Not Designated as Hedging Instruments [Member] | Foreign Exchange Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	1.0	0.9	0.5
Amount of Pre-Tax Gain (Loss) Recognized in AOCL	0.0
Amount of Pre-Tax Gain (Loss) Recognized in AOCL		0.0	0.0
Derivatives Not Designated as Hedging Instruments [Member] | Risk Participation Agreements [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	(0.4)	0.2	0.0
Amount of Pre-Tax Gain (Loss) Recognized in AOCL	0.0
Amount of Pre-Tax Gain (Loss) Recognized in AOCL		0.0	0.0
Designated as Hedging Instrument [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	(0.8)	1.6	6.8
Amount of Pre-Tax Gain (Loss) Recognized in AOCL	1.9
Amount of Pre-Tax Gain (Loss) Recognized in AOCL		(1.7)	(1.0)
Designated as Hedging Instrument [Member] | Cash Flow [Member] | Interest Rate-Lock Commitments on Residential Mortgage Loans [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	0.1	0.1	0.1
Amount of Pre-Tax Gain (Loss) Recognized in AOCL	0.0
Amount of Pre-Tax Gain (Loss) Recognized in AOCL		0.0	0.0
Designated as Hedging Instrument [Member] | Interest Rate Swaps [Member] | Fair Value [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	0.3	2.1	5.9
Amount of Pre-Tax Gain (Loss) Recognized in AOCL	0.0
Amount of Pre-Tax Gain (Loss) Recognized in AOCL		0.0	0.0
Designated as Hedging Instrument [Member] | Interest Rate Swaps [Member] | Cash Flow [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	(1.2)	(0.6)	0.8
Amount of Pre-Tax Gain (Loss) Recognized in AOCL	$ 1.9
Amount of Pre-Tax Gain (Loss) Recognized in AOCL		$ (1.7)	$ (1.0)