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Summary of Significant Accounting Policies - Definite lived Intangible Assets Amortization Expense (Detail)
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Minimum [Member] | Customer Lists and Relationships [Member]      
Finite-Lived Intangible Assets [Line Items]      
Finite-lived intangible asset, useful life 5 years 5 years 5 years
Minimum [Member] | Technology [Member]      
Finite-Lived Intangible Assets [Line Items]      
Finite-lived intangible asset, useful life 2 years 2 years 2 years
Maximum [Member] | Customer Lists and Relationships [Member]      
Finite-Lived Intangible Assets [Line Items]      
Finite-lived intangible asset, useful life 15 years 15 years 15 years
Maximum [Member] | Technology [Member]      
Finite-Lived Intangible Assets [Line Items]      
Finite-lived intangible asset, useful life 7 years 7 years 7 years