Derivative Financial Instruments - Details of Derivative Contracts (Detail) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Zero Cost Collar One [Member]
Derivative [Line Items]
Date of transaction	Jun. 18, 2018	Jun. 22, 2017
Type of derivative	Zero cost collar	Zero cost collar
Total notional amount	$ 20,000,000	$ 20,000,000
Month of settlement, start	2018-09	2018-01
Month of settlement, end	2018-12	2018-02
Zero Cost Collar Two [Member]
Derivative [Line Items]
Date of transaction	Jun. 27, 2018	Sep. 28, 2017
Type of derivative	Zero cost collar	Zero cost collar
Total notional amount	$ 32,000,000	$ 54,000,000
Month of settlement, start	2018-09	2018-01
Month of settlement, end	2019-06	2018-06
Forward [Member]
Derivative [Line Items]
Date of transaction	Jun. 18, 2018	Sep. 28, 2017
Type of derivative	Forward	Forward
Total notional amount	$ 40,000,000	$ 36,000,000
Month of settlement, start	2018-09	2018-01
Month of settlement, end	2018-12	2018-06
Forward One [Member]
Derivative [Line Items]
Date of transaction	Jun. 27, 2018
Type of derivative	Forward
Total notional amount	$ 58,000,000
Month of settlement, start	2018-09
Month of settlement, end	2019-06