Claims Accruals (Details) $ in Thousands		12 Months Ended
		Dec. 31, 2015 USD ($) $ / insurance_claim	Dec. 31, 2016 USD ($)
Liability for Claims and Claims Adjustment Expense [Line Items]
Claims accruals		$ 233,710	$ 246,592
Less: current portion of claims accruals		(84,429)	(80,866)
Claims accruals, less current portion		$ 149,281	165,726
Group Medical Specific Deductible | $ / insurance_claim		500,000
Group Medical Aggregating Individual Deductible | $ / insurance_claim		150,000
Auto and collision liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Claims accruals		$ 123,086	128,643
Workers' compensation liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Claims accruals		92,608	101,232
Owner-operator claims liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Claims accruals		12,304	11,736
Cargo damage liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Claims accruals		5,348	4,695
Other liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Claims accruals	[1]	$ 364	$ 286

[1] Includes group medical liability. Effective January 1, 2015, the Company is fully insured on its group medical benefits, subject to contributed premiums. Prior to January 1, 2015, the Company had a $500 thousand specific deductible with an aggregating individual deductible of $150 thousand of each employee health care claim, as well as commercial insurance for the balance.