Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)		1 Months Ended	3 Months Ended	9 Months Ended
	Mar. 01, 2018	Aug. 31, 2019	Sep. 30, 2020	Sep. 30, 2020
Accounting Standards Update 2016-13 [Member]
Summary of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date			Jan. 01, 2020	Jan. 01, 2020
Change in accounting principle, accounting standards update, adopted			true	true
Change in accounting principle, accounting standards update, immaterial effect			true	true
New Bodily Injury and Property Damage Liability Policy [Member]
Summary of Significant Accounting Policies [Line Items]
New bodily injury and property damage liability policy, effective date	Mar. 01, 2018
New bodily injury and property damage liability policy, term	3 years
Risk retention amount per occurrence under new policy	$ 2,000,000.0
Provisional extended term of policy	1 year
Maximum amount received return on premium availability		$ 5,200,000
Self insurance, per occurrence		$ 10,000,000
Reduction in insurance premium expenses			$ 500,000
Accrued additional premium			$ 0	$ 0
Minimum [Member]
Summary of Significant Accounting Policies [Line Items]
Percentage of revenue derived from transporting				97.00%
Average transit time				1 day
Minimum [Member] | Entire 36 Months [Member] | New Bodily Injury and Property Damage Liability Policy [Member]
Summary of Significant Accounting Policies [Line Items]
Returnable premium for changes in claims	$ 15,600,000
Maximum [Member]
Summary of Significant Accounting Policies [Line Items]
Average transit time				5 days
Maximum [Member] | New Bodily Injury and Property Damage Liability Policy [Member]
Summary of Significant Accounting Policies [Line Items]
Single loss limit	8,000,000.0
Aggregate loss limit for each policy year	24,000,000.0
Aggregate loss limit during policy period	48,000,000.0
Paid losses specified limit to pay additional premium	15,600,000
Additional premium to be paid, if losses are paid by insurer are greater than limit specified	11,000,000.0
Maximum [Member] | First 12 months [Member] | New Bodily Injury and Property Damage Liability Policy [Member]
Summary of Significant Accounting Policies [Line Items]
Returnable premium for changes in claims	$ 5,200,000