Summary of Significant Accounting Principles - Customers with Accounts Receivable Greater than 10% (Detail) - Customer Concentration Risk [Member] - Accounts Receivable [Member]	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Hub City Terminals, Inc. [Member]
Revenue, Major Customer [Line Items]
Customers with accounts receivable greater than 10%	18.00%
Walmart Stores, Inc. [Member]
Revenue, Major Customer [Line Items]
Customers with accounts receivable greater than 10%		15.00%
Caterpillar Inc. [Member]
Revenue, Major Customer [Line Items]
Customers with accounts receivable greater than 10%		13.60%