Derivative Financial Instruments - Details of Derivative Contracts (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Zero Cost Collar One [Member]
Derivative [Line Items]
Date of transaction	Jun. 22, 2017	Nov. 11, 2016
Type of derivative	Zero cost collar
Total notional amount	$ 20,000,000	$ 18,000,000
Month of settlement, start	2018-01	2017-03
Month of settlement, end	2018-02	2017-08
Zero Cost Collar Two [Member]
Derivative [Line Items]
Date of transaction	Sep. 28, 2017
Type of derivative	Zero cost collar
Total notional amount	$ 54,000,000
Month of settlement, start	2018-01
Month of settlement, end	2018-06
Forward [Member]
Derivative [Line Items]
Date of transaction	Sep. 28, 2017
Type of derivative	Forward
Total notional amount	$ 36,000,000
Month of settlement, start	2018-01
Month of settlement, end	2018-06