Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total		Common Class A [Member]	Share Capital	Share Capital Common Class A [Member]	Contributed Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Parent	Non-Controlling Interest
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' equity				$ 59,361		$ 41,765	$ 410,258	$ (681)
Dividends			$ 0.44				$ 0.44
Balance at beginning of year at Dec. 31, 2016										$ 2,631
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of Class A non-voting common stock	$ 6,569				$ 6,595
Repurchase of Class A non-voting common stock for cancellation					(7,464)
Cumulative-effect adjustment from adoption of new accounting update of employee share-based accounting						425
Share-based expense						5,583
Vested employee share plan awards						(11,227)
Net income	22,816						$ 22,816
Dividends paid ($0.44 per share for the years 2012, 2013 & 2014)							(5,836)
Adjustments to Additional Paid in Capital, Other							(314)
Dividends received from non-controlling interest							6
Currency translation adjustment	2,263	[1]						2,263
Less net income (loss) attributable to non-controlling interest, net of tax										184
Dividends paid to non-controlling interest	2,448									(2,448)
SEC Schedule, 12-04, Cash Dividends Paid to Registrant, Subsidiaries and Equity Method Investees										(6)
Balance at end of year at Dec. 31, 2017	$ 523,911									361
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' equity				58,492		36,546	$ 426,930	1,582	$ 523,550
Non-controlling interest										361
Dividends	$ 0.44		0.44				$ 0.44
Issuance of Class A non-voting common stock	$ 724				794
Repurchase of Class A non-voting common stock for cancellation					(5,894)
Cumulative-effect adjustment from adoption of new accounting update of employee share-based accounting						0
Share-based expense						6,061
Vested employee share plan awards						(831)
Net income	28,892						$ 28,892
Dividends paid ($0.44 per share for the years 2012, 2013 & 2014)							(5,833)
Adjustments to Additional Paid in Capital, Other							0
Dividends received from non-controlling interest							0
Currency translation adjustment	(1,417)	[1]						(1,417)
Less net income (loss) attributable to non-controlling interest, net of tax										(16)
Dividends paid to non-controlling interest	372									(345)
SEC Schedule, 12-04, Cash Dividends Paid to Registrant, Subsidiaries and Equity Method Investees										0
Balance at end of year at Dec. 31, 2018										0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' equity	$ 545,322			53,392		41,776	$ 449,989	165	545,322
Non-controlling interest										0
Dividends	$ 0.46		$ 0.46				$ 0.46
Issuance of Class A non-voting common stock	$ 2,192				1,565
Repurchase of Class A non-voting common stock for cancellation					$ (8,400)
Cumulative-effect adjustment from adoption of new accounting update of employee share-based accounting						0
Share-based expense						8,128
Vested employee share plan awards						(2,498)
Net income	52,953						$ 52,953
Dividends paid ($0.44 per share for the years 2012, 2013 & 2014)							(5,944)
Adjustments to Additional Paid in Capital, Other							0
Dividends received from non-controlling interest							0
Currency translation adjustment	1,596	[1]						1,596
Less net income (loss) attributable to non-controlling interest, net of tax										0
Dividends paid to non-controlling interest	0									0
SEC Schedule, 12-04, Cash Dividends Paid to Registrant, Subsidiaries and Equity Method Investees										0
Stockholders' equity	$ 592,722			$ 46,557		$ 47,406	$ 496,998	$ 1,761	$ 592,722
Non-controlling interest										$ 0

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