Retirement Plans - Summary of Plan's Assets Carried at Fair Value (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Total fair value	$ 217.2	$ 254.9	$ 227.4
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value	116.1	158.8
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value	$ 101.1	$ 96.1	$ 91.1