Other Income, Net - Additional Information (Detail) $ in Millions	1 Months Ended	3 Months Ended	6 Months Ended
	Jan. 31, 2018 USD ($)	Mar. 31, 2018 USD ($) Parcel	Jun. 30, 2018 USD ($)
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
Promissory Note | Tremont
Disclosure Other Income Net Additional Information Detail [Line Items]
Principal payment of debt	$ 2.2	$ 2.2