Comprehensive Income - Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
OCI, before reclassifications, after tax amount	$ 79.1	$ (55.0)	$ (8.7)
Reclassification adjustment, after-tax amount	10.8	17.2	22.0
Other comprehensive income (loss), pre-tax amount	117.1	(50.0)	21.6
Other comprehensive income (loss), tax effect	(27.2)	12.2	(8.3)
Total other comprehensive income (loss), net of tax	89.9	(37.8)	13.3
Pension and Other Postretirement Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
OCI, before reclassifications, pre-tax amount	12.1
OCI, before reclassification, tax effect	(2.4)
OCI, before reclassifications, after tax amount	9.7	(24.7)	(2.8)
Reclassification adjustment, pre-tax amount	8.3
Reclassification adjustment, tax effect	(1.7)
Reclassification adjustment, after-tax amount	6.6	6.3	4.3
Other comprehensive income (loss), pre-tax amount	20.4	(24.3)	3.0
Other comprehensive income (loss), tax effect	(4.1)	5.9	(1.5)
Total other comprehensive income (loss), net of tax	16.3	(18.4)	1.5
Net Actuarial Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
OCI, before reclassifications, pre-tax amount		(32.6)	(5.5)
OCI, before reclassification, tax effect		7.9	2.7
OCI, before reclassifications, after tax amount		(24.7)	(2.8)
Reclassification adjustment, pre-tax amount		8.6	9.3
Reclassification adjustment, tax effect		(2.1)	(4.6)
Reclassification adjustment, after-tax amount		6.5	4.7
Other comprehensive income (loss), pre-tax amount		(24.0)	3.8
Other comprehensive income (loss), tax effect		5.8	(1.9)
Total other comprehensive income (loss), net of tax		(18.2)	1.9
Prior Service Credit [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification adjustment, pre-tax amount		(0.3)	(0.8)
Reclassification adjustment, tax effect		0.1	0.4
Reclassification adjustment, after-tax amount		(0.2)	(0.4)
Net Unrealized Gains (Losses) on Debt Securities Available for Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
OCI, before reclassifications, pre-tax amount	89.2	(38.3)	(8.6)
OCI, before reclassification, tax effect	(21.3)	9.3	3.3
OCI, before reclassifications, after tax amount	67.9	(29.0)	(5.3)
Reclassification adjustment, pre-tax amount	(0.1)	9.9	25.4
Reclassification adjustment, tax effect	0.0	(2.4)	(9.4)
Reclassification adjustment, after-tax amount	(0.1)	7.5	16.0
Other comprehensive income (loss), pre-tax amount	89.1	(28.4)	16.8
Other comprehensive income (loss), tax effect	(21.3)	6.9	(6.1)
Total other comprehensive income (loss), net of tax	67.8	(21.5)	10.7
Net Unrealized Gain (Losses) On Debt Securities Transferred To Held To Maturity [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
OCI, before reclassifications, after tax amount	0.0	0.0	0.0
Reclassification adjustment, pre-tax amount	4.6	3.9	3.7
Reclassification adjustment, tax effect	(1.1)	(0.9)	(1.4)
Reclassification adjustment, after-tax amount	3.5	3.0	2.3
Other comprehensive income (loss), pre-tax amount	4.6	3.9	3.7
Other comprehensive income (loss), tax effect	(1.1)	(0.9)	(1.4)
Total other comprehensive income (loss), net of tax	3.5	3.0	2.3
Net Unrealized Gains (Losses) on Derivatives Accounted for as Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
OCI, before reclassifications, pre-tax amount	1.9
OCI, before reclassification, tax effect	(0.4)
OCI, before reclassifications, after tax amount	1.5
Reclassification adjustment, pre-tax amount	1.1
Reclassification adjustment, tax effect	(0.3)
Reclassification adjustment, after-tax amount	0.8
Other comprehensive income (loss), pre-tax amount	3.0
Other comprehensive income (loss), tax effect	(0.7)
Total other comprehensive income (loss), net of tax	$ 2.3
Net Unrealized Gains (Losses) on Derivatives Accounted for as Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
OCI, before reclassifications, pre-tax amount		(1.7)	(1.0)
OCI, before reclassification, tax effect		0.4	0.4
OCI, before reclassifications, after tax amount		(1.3)	(0.6)
Reclassification adjustment, pre-tax amount		0.5	(0.9)
Reclassification adjustment, tax effect		(0.1)	0.3
Reclassification adjustment, after-tax amount		0.4	(0.6)
Other comprehensive income (loss), pre-tax amount		(1.2)	(1.9)
Other comprehensive income (loss), tax effect		0.3	0.7
Total other comprehensive income (loss), net of tax		$ (0.9)	$ (1.2)