Supplemental Cash Flow Information - Changes in Current Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Decrease (increase):
Accounts receivable	$ 12,514	$ 1,263	$ (6,052)
Accounts receivable from related parties	4,271	(666)	(2,104)
Inventories	362	863	4,960
Other current assets	(66)	(1,718)	2,345
Other assets	(137)	(3,248)	(7,642)
Increase (decrease):
Accounts payable	(3,157)	886	2,132
Accounts payable to related parties	(1,853)	14,778	(1,851)
Motor fuel taxes payable	(1,637)	(226)	2,649
Accrued expenses and other current liabilities	1,364	1,708	(1,239)
Other long-term liabilities	(1,645)	(83)	3,837
Changes in operating assets and liabilities, net of acquisitions	$ 10,016	$ 13,557	$ (2,965)