Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Share Capital	Share Capital Class A Stock	Contributed Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Balance at beginning of period at Dec. 31, 2017		$ 58,492		$ 36,546	$ 426,930	$ 1,582
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of Class A non-voting common stock			$ 794
Repurchase of Class A non-voting common stock for cancellation			0
Net Income (Loss) Attributable to Parent	$ 20,631				20,631
Share-based expense				4,647
Vested employee share plan awards				(831)
Dividends paid					(4,373)
Currency translation adjustment	(880)					(880)
Cumulative Effect on Retained Earnings, Net of Tax					339
Balance at end of period at Sep. 30, 2018	543,877	59,286		40,362	$ 443,527	702
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends					$ 0.33
Balance at beginning of period at Jun. 30, 2018		58,825		39,287	$ 439,926	603
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of Class A non-voting common stock			461
Repurchase of Class A non-voting common stock for cancellation			0
Net Income (Loss) Attributable to Parent	5,071				5,071
Share-based expense				1,551
Vested employee share plan awards				(476)
Dividends paid					(1,460)
Currency translation adjustment	99					99
Cumulative Effect on Retained Earnings, Net of Tax					(10)
Balance at end of period at Sep. 30, 2018	543,877	59,286		40,362	$ 443,527	702
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends					$ 0.11
Balance at beginning of period at Dec. 31, 2018		53,392		41,776	$ 449,989	165
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of Class A non-voting common stock			1,565
Repurchase of Class A non-voting common stock for cancellation			(8,400)
Net Income (Loss) Attributable to Parent	27,518
Share-based expense				6,071
Vested employee share plan awards				(2,498)
Dividends paid					(4,408)
Currency translation adjustment	1,457					1,457
Cumulative Effect on Retained Earnings, Net of Tax					0
Balance at end of period at Sep. 30, 2019	566,627	46,557		45,349	$ 473,099	1,622
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends					$ 0.34
Balance at beginning of period at Jun. 30, 2019		48,202		43,626	$ 470,693	1,089
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of Class A non-voting common stock			403
Repurchase of Class A non-voting common stock for cancellation			$ (2,048)
Net Income (Loss) Attributable to Parent	3,949
Share-based expense				2,045
Vested employee share plan awards				(322)
Dividends paid					(1,543)
Currency translation adjustment	533					533
Cumulative Effect on Retained Earnings, Net of Tax					0
Balance at end of period at Sep. 30, 2019	$ 566,627	$ 46,557		$ 45,349	$ 473,099	$ 1,622
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends					$ 0.12