Leases and Commitments - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Jun. 30, 2018	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Loans and Leases Receivable Disclosure [Line Items]
Capital lease obligations			$ 70,800
Current maturities of capital leases			17,292	$ 12,929
Capital leases, amortization expense			5,800	7,400	$ 6,200
Capital lease obligations incurred			42,800	2,600	29,600
Proceeds from operating sale leaseback			5,323	10,980	0
Lessee, operating lease, term of contract	6 months	41 months
Operating leases, rent expense			$ 12,426	11,792	$ 9,444
Minimum
Loans and Leases Receivable Disclosure [Line Items]
Capital lease term			36 months
Maximum
Loans and Leases Receivable Disclosure [Line Items]
Capital lease term			84 months
Capital Lease Obligations | Minimum
Loans and Leases Receivable Disclosure [Line Items]
Interest rate, effective percentage			0.00%
Capital Lease Obligations | Maximum
Loans and Leases Receivable Disclosure [Line Items]
Interest rate, effective percentage			4.08%
Sale-leaseback of Trailers to an Unrelated Party
Loans and Leases Receivable Disclosure [Line Items]
Proceeds from operating sale leaseback	$ 5,300	$ 11,000		$ 2,500
Lessee, operating lease, term of contract		3 years 4 months 28 days		48 months
Sale leaseback transaction, deferred gain, net	$ 1,300	$ 30
Revenue and Non-revenue Equipment
Loans and Leases Receivable Disclosure [Line Items]
Purchase obligation			$ 32,800
Affiliated Entity
Loans and Leases Receivable Disclosure [Line Items]
Operating leases, rent expense			$ 100