Supplemental Cash Flow Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental disclosure of cash flow information
Interest paid	$ 23,958	$ 21,407	$ 19,292
Income taxes paid, net of refunds	2,065	(10,587)	4,909
Noncash investing and financing activity
Capital projects incurred but not yet paid	6,840	3,092	7,507
Purchase of a productive asset through seller-financing	0	0	1,010
Shares issued for acquisition of business	0	0	4,303
Investment merger (decreasing equity method investments and non-controlling interest)	8,360	0	0
Dividends declared not yet paid	$ 4,650	$ 4,493	$ 4,338