Summary of Significant Accounting Policies - Summary of Risk Retention Amounts Per Occurrence (Detail)	36 Months Ended
Dec. 31, 2017 USD ($)
Schedule Of Retention Amounts For Uninsured Claims Per Occurrence Basis [Abstract]
Workers’ compensation	$ 1,000,000
Bodily injury and property damage	2,000,000
Employee medical and hospitalization	400,000
Cargo loss and damage	$ 250,000

Schedule of retention amounts for uninsured claims per occurrence basis.

No definition available.

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Uninsured claims amount retained for cargo loss and damage per occurrence.

No definition available.

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Uninsured claims amount retained for employee hospitalization per occurrence.

No definition available.

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Uninsured claims amount retained for injuries and property damages per occurrence.

No definition available.

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Uninsured claims amount retained for workmen compensation liability per occurrence.

No definition available.

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