Summary of Significant Accounting Policies - Summary of Changes in Contract Liabilities (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Contract liabilities:
Deferred revenue, Balance at Beginning of Period	$ 55,932	$ 59,868
Deferred revenue, Additions	2,000	2,000
Deferred revenue, Deductions	(15,772)	(11,009)
Deferred revenue, Balance at End of Period	$ 42,160	$ 50,859