Stockholders' Equity - Summary of Amounts Reclassified from AOCL (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other non-interest expense	$ 27.5	$ 34.4
Interest expense - notes and debentures (interest rate locks are less than $0.1 million)	8.8	8.8
Income before income tax expense	(166.2)	(144.6)
Income tax expense	35.8	30.0
Net Income (Loss) Attributable to Parent	(130.4)	(114.6)
Reclassification out of AOCL
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net Income (Loss) Attributable to Parent	(2.6)	(2.7)
Reclassification out of AOCL | Net Actuarial Loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other non-interest expense	(2.0)	(1.3)
Reclassification out of AOCL | Pension and Other Postretirement Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income tax expense	(2.0)	(1.3)
Income tax expense	0.4	(0.5)
Net Income (Loss) Attributable to Parent	(1.6)	(1.8)
Reclassification out of AOCL | Net Unrealized Gains (Losses) on Debt Securities Available-for-Sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income tax expense	0.0	0.0
Income tax expense	0.0	0.0
Net Income (Loss) Attributable to Parent	0.0	0.0
Reclassification out of AOCL | Net Unrealized Gains (Losses) on Debt Securities Transferred to Held-to-Maturity
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income tax expense	(1.3)	(0.8)
Income tax expense	0.3	0.2
Net Income (Loss) Attributable to Parent	(1.0)	(0.6)
Reclassification out of AOCL | Net Unrealized Gains (Losses) on Derivatives Accounted for as Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income tax expense	0.0	(0.4)
Income tax expense	0.0	0.1
Net Income (Loss) Attributable to Parent	0.0	(0.3)
Reclassification out of AOCL | Net Unrealized Gains (Losses) on Derivatives Accounted for as Cash Flow Hedges | Interest rate swaps
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest expense - notes and debentures (interest rate locks are less than $0.1 million)	0.0	(0.4)
Reclassification out of AOCL | Net Unrealized Gains (Losses) on Derivatives Accounted for as Cash Flow Hedges | Interest rate locks
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest expense - notes and debentures (interest rate locks are less than $0.1 million)	0.0	0.0
Maximum | Reclassification out of AOCL | Net Unrealized Gains (Losses) on Derivatives Accounted for as Cash Flow Hedges | Interest rate swaps
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net Income (Loss) Attributable to Parent	(0.1)	(0.1)
Maximum | Reclassification out of AOCL | Net Unrealized Gains (Losses) on Derivatives Accounted for as Cash Flow Hedges | Interest rate locks
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net Income (Loss) Attributable to Parent	$ (0.1)	$ (0.1)