Retirement Plans - Summary of Plan's Assets Carried at Fair Value (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Total fair value	$ 219.4	$ 207.8	$ 207.5
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value	107.5	108.1
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value	$ 111.9	$ 99.7	$ 100.7