Schedule of Lease payments to the Mann foundation (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Mann Foundation | Sublease Agreement
Lease And Rental Expense [Line Items]
Lease payments	$ 40	$ 67	$ 161	$ 200