Condensed Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Common stock [Member]	Additional paid-in capital ("APIC") [Member]	Retained earnings [Member]	Accumulated other comprehensive loss [Member]	Non-controlling interest ("NCI") [Member]	Contingently redeemable non-controlling interest [Member] Contingently Redeemable Performance Share Units [Member]	Total
Balance at Dec. 31, 2017	$ 138,582	$ 41,005	$ 602,609	$ (42,514)	$ 5,069	$ 9,014	$ 744,751
Balance, shares at Dec. 31, 2017	107,269,783
Net income			17,138		69		17,207
Other comprehensive loss				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 fair 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	16,576	757,450
Balance, shares at Mar. 31, 2018	107,471,895
Balance at Dec. 31, 2018	$ 181,780	56,885	648,255	(56,277)	5,067	923	835,710
Balance, shares at Dec. 31, 2018	108,682,030
Net income			18,164		8		18,172
Other comprehensive loss				(1,622)	(12)		(1,634)
Comprehensive income			18,164	(1,622)	(4)		16,538
Stock option exercises	$ 2,478	(850)					$ 1,628
Stock option exercises, shares	82,926						82,926
Issuance of common stock related to vesting of share units	$ 5,039	(10,064)					$ (5,025)
Issuance of common stock related to vesting of share units, shares	193,950
Stock option compensation expense		1,539					1,539
Equity-classified PSU expense		2,322				46	2,322
Equity-classified PSU dividend equivalents		222	(237)			15	(15)
Cash dividends paid			(19,568)				(19,568)
Balance at Mar. 31, 2019	$ 189,297	$ 50,054	$ 646,614	$ (57,899)	$ 5,063	$ 984	$ 833,129
Balance, shares at Mar. 31, 2019	108,958,906