Retirement Plans - Reconciliation of Beginning and Ending Balances for Level 3 Assets of Plan Measured at Fair Value (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year	$ 220.0	$ 219.4
Fair value of plan assets at end of year	243.7	220.0
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year	94.9	111.9
Settlements		(9.6)
Unrealized gain (loss)	13.0	(7.4)
Fair value of plan assets at end of year	$ 107.9	$ 94.9