Components of Other Intangible Acquired (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Other Intangible Assets [Line Items]
Intangible Assets, Gross Amount	$ 86,943	$ 20,543
Finite-lived intangible assets, Accumulated Amortization	$ 12,595	$ 8,699
Estenson Logistics, LLC [Member]
Other Intangible Assets [Line Items]
Intangible assets estimated useful life	15 years
Estenson Logistics, LLC [Member] | Customer Relationships [Member]
Other Intangible Assets [Line Items]
Finite-lived intangible assets, Gross Amount	$ 66,000
Finite-lived intangible assets, Accumulated Amortization	2,200
Finite-lived intangible assets, Net Carrying Value	$ 63,800
Intangible assets estimated useful life	15 years
Estenson Logistics, LLC [Member] | Trade Name [Member]
Other Intangible Assets [Line Items]
Finite-lived intangible assets, Gross Amount	$ 400
Finite-lived intangible assets, Accumulated Amortization	400
Finite-lived intangible assets, Net Carrying Value	$ 0
Intangible assets estimated useful life	3 months