Comprehensive Income - Summary of Amounts Reclassified from AOCL (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other non-interest income									$ 85.9	$ 50.9	$ 62.6
Interest and dividend income - securities									(34.9)	(33.3)	(29.9)
Income before income tax expense	$ 173.9	$ 165.5	$ 168.4	$ 144.6	$ 148.7	$ 149.2	$ 141.0	$ 137.3	652.4	576.3	467.1
Income tax expense	(36.4)	(30.4)	(35.2)	(30.0)	(15.8)	(32.2)	(30.8)	(29.4)	(132.0)	(108.2)	(129.9)
Net income	$ 137.5	$ 135.1	$ 133.2	$ 114.6	$ 132.9	$ 117.0	$ 110.2	$ 107.9	520.4	468.1	337.2
Total reclassifications for the period									10.8	17.2	22.0
Pension and Other Postretirement Plans [Member] | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income tax expense									(8.3)	(8.3)	(8.5)
Income tax expense									1.7	2.0	4.2
Net income									(6.6)	(6.3)	(4.3)
Net Actuarial Loss [Member] | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other non-interest income									(8.3)	(8.6)	(9.3)
Prior Service Credit [Member] | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other non-interest income									0.0	0.3	0.8
Net Unrealized Gains (Losses) on Debt Securities Available for Sale [Member] | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income tax expense									0.1	(9.9)	(25.4)
Income tax expense									0.0	2.4	9.4
Net income									0.1	(7.5)	(16.0)
Net Unrealized Gain (Losses) On Debt Securities Transferred To Held To Maturity [Member] | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income tax expense									(4.6)	(3.9)	(3.7)
Income tax expense									1.1	0.9	1.4
Net income									(3.5)	(3.0)	(2.3)
Net Unrealized Gains (Losses) on Derivatives Accounted for as Cash Flow Hedges [Member] | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income tax expense									(1.1)
Income tax expense									0.3
Net income									(0.8)
Net Unrealized Gains (Losses) on Derivatives Accounted for as Cash Flow Hedges [Member] | Interest Rate Swaps [Member] | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and dividend income - securities									(1.2)
Net Unrealized Gains (Losses) on Derivatives Accounted for as Cash Flow Hedges [Member] | Interest Rate Locks [Member] | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and dividend income - securities									$ 0.1
Net Unrealized Gains (Losses) on Derivatives Accounted for as Cash Flow Hedges [Member] | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income tax expense										(0.5)	0.9
Income tax expense										0.1	(0.3)
Net income										(0.4)	0.6
Net Unrealized Gains (Losses) on Derivatives Accounted for as Cash Flow Hedges [Member] | Interest Rate Swaps [Member] | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and dividend income - securities										(0.6)	0.8
Net Unrealized Gains (Losses) on Derivatives Accounted for as Cash Flow Hedges [Member] | Interest Rate Locks [Member] | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and dividend income - securities										$ 0.1	$ 0.1