Changes in Contract Liabilities, which Classified as Unearned Income in Accrued Liabilities or Deferred Revenue in Long-term Liabilities (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Contract With Customer Asset And Liability [Abstract]
Unearned income	$ 1,138	$ 510
Deferred revenue	6,182	6,757
Contract liabilities from contracts with customers	7,320	$ 7,267
Unearned income, change	628
Deferred revenue, change	(575)
Contract liabilities from contracts with customers, change	$ 53