Employee Benefit Plans - Changes in Benefit Obligations and Plan Assets (Detail) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Feb. 28, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair value of plan assets:
Employer contributions	$ 50.0
Pension Plans [Member]
Benefit Obligation
Beginning of year		$ 562.7	$ 584.8
Service cost		0.0	0.0
Interest cost		22.9	20.2	$ 19.1
Actuarial loss (gain)		80.2	(51.0)
Benefits paid		(28.2)	(21.4)
Settlements		(13.1)	(2.7)
Acquisitions		41.0	35.2
Plan revaluations		0.0	(2.4)
End of year		665.5	562.7	584.8
Fair value of plan assets:
Beginning of year		596.4	584.9
Actual return on assets		138.6	(40.9)
Employer contributions		18.7	52.7
Benefits paid		(28.2)	(21.4)
Settlements		(13.1)	(2.7)
Acquisitions		34.3	23.8
End of year		746.7	596.4	584.9
Funded status at end of year		81.2	33.7
Amounts recognized in the Consolidated Statements of Condition:
Other assets		121.4	78.0
Other liabilities		(40.2)	(44.3)
Funded status at end of year		81.2	33.7
Other Postretirement Plans [Member]
Benefit Obligation
Beginning of year		15.3	14.6
Service cost		0.3	0.3
Interest cost		0.6	0.5
Actuarial loss (gain)		1.2	(1.7)
Benefits paid		(0.7)	(0.8)
Settlements		0.0	0.0
Acquisitions		2.0	2.4
Plan revaluations		0.0	0.0
End of year		18.7	15.3	14.6
Fair value of plan assets:
Beginning of year		0.0	0.0
Actual return on assets		0.0	0.0
Employer contributions		0.7	0.8
Benefits paid		(0.7)	(0.8)
Settlements		0.0	0.0
Acquisitions		0.0	0.0
End of year		0.0	0.0	$ 0.0
Funded status at end of year		(18.7)	(15.3)
Amounts recognized in the Consolidated Statements of Condition:
Other assets		0.0	0.0
Other liabilities		(18.7)	(15.3)
Funded status at end of year		$ (18.7)	$ (15.3)