Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands			3 Months Ended				12 Months Ended
	Oct. 18, 2016	Feb. 05, 2016	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Mar. 31, 2016	Dec. 31, 2016	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2015
Loss Contingencies [Line Items]
Restricted cash							$ 18,251
Other Current Assets [Member]
Loss Contingencies [Line Items]
Prepayments to suppliers								$ 8,926	$ 7,404
Shareholder Derivative Actions [Member]
Loss Contingencies [Line Items]
Attorneys' fee and litigation expenses	$ 750
Settlement charge paid							2,258
Proceeds from insurance settlement						$ 3,000
Insurance proceeds in restricted cash reclassified from other receivables							(3,078)
Settlement charge paid, portion excluding interest							$ 2,250
Securities Class Action Complaints [Member]
Loss Contingencies [Line Items]
Damages sought		$ 23,500
Proceeds from insurance settlement						29,571
Insurance proceeds in restricted cash reclassified from other receivables						$ 29,571
Disbursements of accrued claims and settlements					$ 23,500
Restricted cash					$ 6,114
Securities Class Action Complaints [Member] | Accrued Expense [Member]
Loss Contingencies [Line Items]
Accrued claim settlement										$ 23,500
Securities Class Action Complaints [Member] | Other Receivables [Member]
Loss Contingencies [Line Items]
Insurance receivable										$ 29,571
Securities and Exchange Commission Actions [Member]
Loss Contingencies [Line Items]
Attorneys' fee and litigation expenses				$ 3,000
Settlement charge paid			$ 3,000