Supplemental Cash Flow Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Supplemental disclosure of cash flow information
Interest paid	$ 29,607	$ 23,958	$ 21,407
Income taxes paid (refunded), net	(5,439)	2,065	(10,587)
Noncash investing and financing activity
Debt resulting from accounting standard adoption	36,953	0	0
Railcar assets resulting from accounting standard adoption	25,643	0	0
Capital projects incurred but not yet paid	14,165	6,840	3,092
Dividends declared not yet paid	5,515	4,650	4,493
Debt financing fees incurred but not yet paid	2,288	0	0
Investment merger (decreasing equity method investments and non-controlling interest)	$ 0	$ 8,360	$ 0