Summary of Significant Accounting Policies - Summary of Changes in Contract Liabilities (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)
Contract liabilities:
Deferred revenue, Balance at Beginning of Period	$ 59,868
Deferred revenue, Additions	3,000
Deferred revenue, Deductions	(16,604)
Deferred revenue, Balance at End of Period	$ 46,264