Derivative Financial Instruments - Details of Derivative Contracts (Detail) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2017
Zero Cost Collar One [Member]
Derivative [Line Items]
Date of transaction	Jun. 22, 2017	Nov. 11, 2016
Total notional amount	$ 65,000,000	$ 18,000,000
Month of settlement, start	2017-10	2017-03
Month of settlement, end	2018-02	2017-08
Forward [Member]
Derivative [Line Items]
Date of transaction	Jun. 22, 2017
Total notional amount	$ 15,000,000
Month of settlement, start	2017-10
Month of settlement, end	2017-12
Zero Cost Collar Two [Member]
Derivative [Line Items]
Date of transaction	Sep. 28, 2017
Total notional amount	$ 54,000,000
Month of settlement, start	2018-01
Month of settlement, end	2018-06
Forward One [Member]
Derivative [Line Items]
Date of transaction	Sep. 28, 2017
Total notional amount	$ 36,000,000
Month of settlement, start	2018-01
Month of settlement, end	2018-06