Comprehensive Income - Deferred Income Taxes Applicable to Components of Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI Tax, Attributable to Parent	$ 52.2	$ 79.4	$ 105.5
Pension and Other Postretirement Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI Tax, Attributable to Parent	55.3	59.4	83.5
Net Unrealized Gains (Losses) on Debt Securities Available for Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI Tax, Attributable to Parent	(6.6)	14.7	12.7
Net Unrealized Gain (Losses) On Debt Securities Transferred To Held To Maturity [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI Tax, Attributable to Parent	3.6	4.7	8.8
Net Unrealized Gains (Losses) on Derivatives Accounted for as Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI Tax, Attributable to Parent	$ (0.1)
Net Unrealized Gains (Losses) on Derivatives Accounted for as Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI Tax, Attributable to Parent		$ 0.6	$ 0.5