Geographic information: (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013 Segment	Dec. 31, 2012	Dec. 31, 2011
Geographic information:
Number of operating segments									1
Geographic information
Revenues	$ 89,859	$ 87,761	$ 85,803	$ 84,553	$ 82,612	$ 79,656	$ 77,817	$ 76,888	$ 347,979	$ 316,973	$ 305,500
Long lived assets, net	341,231				311,222				341,231	311,222
North America
Geographic information
Revenues									274,320	253,396	240,116
Long lived assets, net	251,352				225,060				251,352	225,060
Europe
Geographic information
Revenues									73,659	63,577	65,384
Long lived assets, net	$ 89,879				$ 86,162				$ 89,879	$ 86,162