Description of the business and recent developments: Stockholder's deficit (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Increase (Decrease) in Stockholders' Equity
Balance	$ (222,225,000)	$ (164,231,000)	$ (203,679,000)	$ (149,002,000)
Balance (in shares)			46,840,434
Equity-based compensation	6,666,000	5,714,000	$ 12,224,000	9,484,000
Foreign currency translation	2,913,000	1,786,000	(580,000)	(39,000)
Issuances of common stock		1,000		1,000
Exercises of options	270,000	745,000	989,000	919,000
Dividends paid	(31,738,000)	(27,741,000)	(62,295,000)	(54,306,000)
Net income	8,564,000	7,136,000	17,791,000	16,353,000
Balance	$ (235,550,000)	(176,590,000)	$ (235,550,000)	(176,590,000)
Balance (in shares)	47,279,201		47,279,201
Common Stock
Increase (Decrease) in Stockholders' Equity
Balance	$ 47,000	$ 46,000	$ 47,000	$ 46,000
Balance (in shares)	47,139,369	46,350,434	46,840,434	46,336,499
Forfeitures of shares granted to employees (in shares)	(972)	(1,702)	(37,280)	(3,886)
Issuances of common stock		$ 1,000		$ 1,000
Issuances of common stock (in shares)	135,280	438,478	455,030	448,978
Exercises of options (in shares)	5,524	19,160	21,017	24,779
Balance	$ 47,000	$ 47,000	$ 47,000	$ 47,000
Balance (in shares)	47,279,201	46,806,370	47,279,201	46,806,370
Additional Paid-in Capital
Increase (Decrease) in Stockholders' Equity
Balance	$ 499,455,000	$ 475,275,000	$ 493,178,000	$ 471,331,000
Equity-based compensation	6,666,000	5,714,000	12,224,000	9,484,000
Exercises of options	270,000	745,000	989,000	919,000
Balance	506,391,000	481,734,000	506,391,000	481,734,000
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity
Balance	(15,819,000)	(12,753,000)	(12,326,000)	(10,928,000)
Foreign currency translation	2,913,000	1,786,000	(580,000)	(39,000)
Balance	(12,906,000)	(10,967,000)	(12,906,000)	(10,967,000)
Accumulated Deficit
Increase (Decrease) in Stockholders' Equity
Balance	(705,908,000)	(626,799,000)	(684,578,000)	(609,451,000)
Dividends paid	(31,738,000)	(27,741,000)	(62,295,000)	(54,306,000)
Net income	8,564,000	7,136,000	17,791,000	16,353,000
Balance	$ (729,082,000)	$ (647,404,000)	$ (729,082,000)	$ (647,404,000)