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]	Non-controlling interest [Member]	Contingently redeemable non-controlling interest [Member] Contingently Redeemable Performance Share Units [Member]	Total
Balance at Dec. 31, 2015	$ 131,530	$ 27,728	$ 601,051	$ (57,133)	$ 4,183	$ 24,785		$ 707,359
Balance, shares at Dec. 31, 2015	107,200,470
Net income			91,832		346	1,334		92,178
Other comprehensive income (loss)				(9,993)	(23)	169		(10,016)
Comprehensive income			91,832	(9,993)	323	1,503		82,162
Change in value of contingently redeemable NCI			(21,186)			21,186		(21,186)
Stock option exercises	$ 30,670	(6,332)						24,338
Stock option exercises, shares	1,081,531
Stock option tax adjustment		443						443
Stock option compensation expense		5,507						5,507
Modification of PSUs			(70)				$ 2,175	(70)
Equity-classified PSU expense		283					1,698	283
Equity-classified PSU dividend equivalents		9	(62)				42	(53)
Change in value of contingently redeemable PSUs			(35)				35	(35)
NCI acquired in a business combination					596			596
Acquisition of NCI					(226)	(44,141)		(226)
Shares repurchased	$ (36,726)							$ (36,726)
Shares repurchased, shares	(1,460,000)							(1,460,000)
Cash dividends paid			(70,459)		(103)	(3,333)		$ (70,562)
Balance at Dec. 31, 2016	$ 125,474	27,638	601,071	(67,126)	4,773		3,950	691,830
Balance, shares at Dec. 31, 2016	106,822,001
Net income			75,027		279			75,306
Other comprehensive income (loss)				24,612	58			24,670
Comprehensive income			75,027	24,612	337			99,976
Stock option exercises	$ 13,017	(3,081)						9,936
Stock option exercises, shares	444,571
Stock option compensation expense		13,700						13,700
Assumption of stock options on acquisition of IronPlanet		2,330						2,330
Settlement of equity-classified PSUs	$ 91						(172)	91
Settlement of equity-classified PSUs, shares	3,211
Modification of PSUs			(382)				1,803	(382)
Equity-classified PSU expense		340					3,189	340
Equity-classified PSU dividend equivalents		78	(227)				149	(149)
Change in value of contingently redeemable PSUs			(95)				95	(95)
NCI acquired in a business combination
Acquisition of NCI
Shares repurchased, shares								0
Cash dividends paid			(72,785)		(41)			$ (72,826)
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
Contingently redeemable Performance share units, Balance at Dec. 31, 2017								9,014
Net income			121,479		27			121,506
Other comprehensive income (loss)				(13,763)	(29)			(13,792)
Comprehensive income			121,479	(13,763)	(2)			107,714
Stock option exercises	$ 37,308	(8,784)						28,524
Stock option exercises, shares	1,235,154
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		8,252						8,252
Modification of PSUs		12,365	958				(6,622)	13,323
Equity-classified PSU expense		5,384					5,872	5,384
Equity-classified PSU dividend equivalents		325	(678)				353	(353)
Change in value of contingently redeemable PSUs			(109)				109	(109)
Cash dividends paid			(75,678)					(75,678)
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