Revenue - Summary of Asset Balances and Related Amortization Expense For Contract Costs (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018
Costs to Obtain a Contract [Member]
Capitalized Contract Cost [Line Items]
Beginning Balance	$ 136,911	$ 126,207
Capitalization of Costs	7,848	22,970
Amortization	(4,640)	(9,058)
Ending Balance	140,119	140,119
Costs to Fulfill a Contract [Member]
Capitalized Contract Cost [Line Items]
Beginning Balance	80,589	72,061
Capitalization of Costs	9,382	20,425
Amortization	(2,772)	(5,287)
Ending Balance	$ 87,199	$ 87,199