Environmental Settlement and Related Costs - Additional Information (Detail)	Apr. 08, 2016 USD ($)	Jul. 20, 2015 USD ($)	Oct. 01, 2014 USD ($) Charge	Apr. 21, 2017 USD ($)	Aug. 25, 2014 USD ($) Installment
Environmental Exit Cost [Line Items]
Fine agreed to pay for violation of the Clean Water Act			$ 600,000
Period of probation			2 years
Number of misdemeanor charges to which the company pleaded guilty | Charge			3
Civil penalties per violation					$ 3,000,000
Number of installments for payment of civil penalty | Installment					2
Estimated site restoration costs					$ 13,000,000
First Installment [Member]
Environmental Exit Cost [Line Items]
Civil penalty payment plus interest		$ 1,000,000
Subsequent Event [Member] | Second Installment [Member]
Environmental Exit Cost [Line Items]
Civil penalty payment plus interest	$ 250,000
Scenario, Forecast [Member] | Third Installment [Member]
Environmental Exit Cost [Line Items]
Civil penalty payment due plus interest				$ 1,750,000