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Cash, Cash Equivalents and Restricted Cash - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	$ 48,274	$ 58,215	$ 54,408	$ 64,765
Net cash outflow	13,890	11,382
Cash and cash equivalents	48,097		54,291
Accounting Standards Update 2016-18 [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Net cash outflow		11,382
Previously Reported [Member] \| Accounting Standards Update 2016-18 [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Net cash outflow		11,299
Prepaid Expenses and Other Current Assets [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	$ 177		$ 117
Cash [Member] \| Accounting Standards Update 2016-18 [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash		$ 72		$ 155