Recent Accounting Standards - Additional Information (Detail) - USD ($)	Sep. 30, 2018	Jan. 01, 2018	Jan. 01, 2016
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Capitalized cost amortization period	3 years
ASU 2014-09
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Capitalized cost amortization period	3 years
ASU 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cumulative-effect adjustment to retained earnings			$ 13,000,000
ASU 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606 | Deferred Revenue
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cumulative-effect adjustment to retained earnings			(12,900,000)
ASU 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606 | Deferred Tax Assets Net of Deferred Tax Liabilities
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cumulative-effect adjustment to retained earnings			5,200,000
ASU 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606 | Other Non-current Assets
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cumulative-effect adjustment to retained earnings			(4,400,000)
ASU 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606 | Other Current Assets
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cumulative-effect adjustment to retained earnings			$ (900,000)
Accounting Standards Update No. 2016-16
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cumulative-effect adjustment to retained earnings		$ 0