Commitments and Contingencies - Additional Information (Detail) - Procurement Agreement [Member] - Europe [Member] - Airtime [Member] - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2001
Long Term Purchase Commitment [Line Items]
Credits provided					$ 3,736
Credits used	$ 8	$ 8	$ 22	$ 23
Unused credits granted	$ 1,955	$ 1,986	$ 1,955	$ 1,986