Goodwill and Intangible Assets - Classes of Intangible Assets (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Cost	$ 85,094	$ 88,094
Accumulated Amortization	15,185	13,929
Net	69,909	74,165
Customer Relationships
Finite-Lived Intangible Assets [Line Items]
Cost	85,068	85,068
Accumulated Amortization	15,171	10,925
Net	$ 69,897	$ 74,143
Non-Compete Agreement
Finite-Lived Intangible Assets [Line Items]
Amortization Period	36 months	36 months
Cost	$ 26	$ 26
Accumulated Amortization	14	4
Net	$ 12	$ 22
Trade Name
Finite-Lived Intangible Assets [Line Items]
Amortization Period		24 months
Cost		$ 3,000
Accumulated Amortization		3,000
Net		$ 0
Minimum | Customer Relationships
Finite-Lived Intangible Assets [Line Items]
Amortization Period	156 months	156 months
Maximum | Customer Relationships
Finite-Lived Intangible Assets [Line Items]
Amortization Period	180 months	180 months