Retirement Plans - Summary of Plan's Assets Carried at Fair Value (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Total fair value	$ 220.0	$ 219.4	$ 207.8
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value	125.1	107.5
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value	$ 94.9	$ 111.9	$ 99.7