Other Income, Net - Additional Information (Detail) $ in Millions		1 Months Ended	3 Months Ended		9 Months Ended
	May 30, 2018 USD ($)	Jan. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Mar. 31, 2018 USD ($) Parcel	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)
Disclosure Other Income Net Additional Information Detail [Line Items]
Gain on securities transaction recognized					$ 12.6	$ 0.1
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
Promissory Note | Tremont
Disclosure Other Income Net Additional Information Detail [Line Items]
Principal payment of debt		$ 2.2		2.2
Snake River
Disclosure Other Income Net Additional Information Detail [Line Items]
Gain on securities transaction recognized	$ 12.5		$ 12.5		$ 12.5
NL
Disclosure Other Income Net Additional Information Detail [Line Items]
Net proceeds				$ 0.6