Other Income, Net - Additional Information (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) Parcel
Disclosure Other Income Net Additional Information Detail [Line Items]
Number of land parecels | Parcel	2
Net proceeds	$ 19.5
Pre-tax gain on sale of land	11.9
Tremont
Disclosure Other Income Net Additional Information Detail [Line Items]
Net proceeds	18.9
NL
Disclosure Other Income Net Additional Information Detail [Line Items]
Net proceeds	$ 0.6