Debt - Summary of total debt (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Total debt	$ 178,190	$ 89,792
Less: Current portion of long-term debt	(6,552)	(3,681)
Long-term debt	171,638	86,111
Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Total debt	$ 52,217	31,486
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Term (years)	5 years
Total debt	$ 125,973	58,306
Equipment Note One [Member] | Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Stated Interest Rate (per annum)	4.16%
Term (years)	5 years
Total debt	$ 10,793	12,655
Equipment Note Two [Member] | Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Stated Interest Rate (per annum)	4.23%
Term (years)	7 years
Total debt	$ 11,200	12,279
Equipment Note Three [Member] | Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Stated Interest Rate (per annum)	3.97%
Term (years)	5 years
Total debt	$ 2,124	2,291
Equipment Note Four [Member] | Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Stated Interest Rate (per annum)	4.02%
Term (years)	7 years
Total debt	$ 2,211	2,313
Equipment Note Five [Member] | Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Stated Interest Rate (per annum)	4.01%
Term (years)	7 years
Total debt	$ 1,851	1,948
Equipment Note Six [Member] | Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Stated Interest Rate (per annum)	2.89%
Term (years)	7 years
Total debt	$ 15,005	0
Equipment Note Seven [Member] | Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Stated Interest Rate (per annum)	3.09%
Term (years)	5 years
Total debt	$ 9,033	$ 0