Condensed Consolidated Statements of Changes in Equity - 3 months ended Mar. 31, 2018 - USD ($) $ in Thousands	Common stock [Member]	Additional paid-in capital ("APIC") [Member]	Retained earnings [Member]	Accumulated other comprehensive income (loss) [Member]	Non-controlling interest ("NCI") [Member]	Performance Share Units [Member]	Total
Balance at Dec. 31, 2017	$ 138,582	$ 41,005	$ 602,609	$ (42,514)	$ 5,069		$ 744,751
Balance, shares at Dec. 31, 2017	107,269,783
Contingently redeemable Performance share units, Balance at Dec. 31, 2017						$ 9,014	9,014
Net income			17,138		69		17,207
Other comprehensive income				4,895	12		4,907
Comprehensive income			17,138	4,895	81		22,114
Stock option exercises	$ 5,805	(1,492)					4,313
Stock option exercises, shares	202,112
Stock option compensation expense		2,343					2,343
Modification of PSUs		703	(134)			6,132	569
Equity-classified PSU expense		1,678				1,357	1,678
Equity-classified PSU dividend equivalents		90	(200)			110	(110)
Change in value of contingently redeemable PSUs			37			(37)	37
Cash dividends paid			(18,245)				(18,245)
Balance at Mar. 31, 2018	$ 144,387	$ 44,327	$ 601,205	$ (37,619)	$ 5,150		757,450
Balance, shares at Mar. 31, 2018	107,471,895
Contingently redeemable Performance share units, Balance at Mar. 31, 2018						$ 16,576	$ 16,576