Summary of Significant Accounting Policies - Summary of Changes in Contract Liabilities (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Contract liabilities:
Deferred revenue, Balance at Beginning of Period	$ 59,868
Deferred revenue, Additions	2,000
Deferred revenue, Deductions	(11,009)
Deferred revenue, Balance at End of Period	$ 50,859