Condensed Consolidated Statements of Changes in Equity - 9 months ended Sep. 30, 2018 - 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 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			85,993		53		86,046
Other comprehensive loss				(7,759)	(22)		(7,781)
Comprehensive income			85,993	(7,759)	31		78,265
Stock option exercises	$ 34,876	(7,804)					$ 27,072
Stock option exercises, shares	1,154,541						1,154,541
Issuance of common stock related to vesting of share units	$ 5,890	(1,662)	(326)			(7,803)	$ 3,902
Issuance of common stock related to vesting of share units, shares	177,093
Stock option compensation expense		6,711					6,711
Modification of PSUs		12,365	958			(6,622)	13,323
Equity-classified PSU expense		3,153				5,826	3,153
Equity-classified PSU dividend equivalents		173	(514)			341	(341)
Change in fair value of contingently redeemable PSUs			(108)			108	(108)
Cash dividends paid			(56,116)				(56,116)
Balance at Sep. 30, 2018	$ 179,348	$ 53,941	$ 632,496	$ (50,273)	$ 5,100		820,612
Balance, shares at Sep. 30, 2018	108,601,417
Contingently redeemable Performance share units, Balance at Sep. 30, 2018						$ 864	$ 864