Segment information: (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 Segment	Mar. 31, 2012
Segment information:
Number of operating segments	1
Geographic information:
Service revenue	$ 84,553	$ 76,888
Long lived assets, net	328,123	311,222
North America
Geographic information:
Service revenue	66,691	61,745
Long lived assets, net	245,572	225,060
Europe
Geographic information:
Service revenue	17,862	15,143
Long lived assets, net	$ 82,551	$ 86,162