Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)		3 Months Ended		9 Months Ended
	Mar. 01, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
New Auto Liability Policy [Member]
Summary of Significant Accounting Policies [Line Items]
New auto liability policy, effective date	Mar. 01, 2018
New auto liability policy, term	3 years
Risk retention amount per occurrence remains under new policy	$ 2,000,000
Provisional extended term of policy	12 months
Accrued additional premium		$ 0		$ 0
Policy description				Effective March 1, 2018, the Company entered into a new auto liability policy with a three-year term. The risk retention amount per occurrence remains at $2.0 million under the new policy. The policy includes a limit for a single loss of $8.0 million, an aggregate loss limit of $24.0 million for each policy year, and a $48.0 million aggregate loss limit for the 36 month term ended March 1, 2021. The policy includes a returnable premium of up to $5.2 million, to be adjusted by the insurer for changes in claims, and a provision to extend the term of the policy for one additional 12 month period, if management and the insurer mutually agree to commute the policy for the first 12 months of the policy term. A decision with respect to commutation of the first 12 months of the policy cannot be made before March 1, 2019. The policy also includes a returnable premium of up to $15.6 million, to be adjusted by the insurer for changes in claims, if management and the insurer mutually agree to commute the policy for the entire 36 months. A decision with respect to commutation of the entire policy cannot be made before August 30, 2021, unless both the Company and the insurance carrier agree to a commutation prior to the end of the policy term. Additionally, the Company may be required to pay an additional premium of up to $11.0 million if paid losses are greater than $15.6 million over the three year policy period.
Minimum [Member]
Summary of Significant Accounting Policies [Line Items]
Percentage of revenue derived from transporting		97.60%	97.20%	97.60%	97.30%
Average transit time				1 day
Minimum [Member] | New Auto Liability Policy [Member]
Summary of Significant Accounting Policies [Line Items]
Paid losses specified limit to pay additional premium	$ 15,600,000
Maximum [Member]
Summary of Significant Accounting Policies [Line Items]
Average transit time				5 days
Payment terms		30 days		30 days
Maximum [Member] | New Auto Liability Policy [Member]
Summary of Significant Accounting Policies [Line Items]
Single loss limit	8,000,000
Aggregate loss limit for each policy year	24,000,000
Aggregate loss limit during policy period	48,000,000
Additional premium to be paid, if paid losses are greater than limit specified	11,000,000
Maximum [Member] | New Auto Liability Policy [Member] | First 12 months [Member]
Summary of Significant Accounting Policies [Line Items]
Returnable premium for changes in claims	5,200,000
Maximum [Member] | New Auto Liability Policy [Member] | Entire 36 Months [Member]
Summary of Significant Accounting Policies [Line Items]
Returnable premium for changes in claims	$ 15,600,000