Income taxes (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
U.S. Federal:
Current	$ 1,651	$ 1,971	$ 214
Deferred	1,189	2,261	(10,360)
Total U.S. Federal	2,840	4,232	(10,146)
U.S. state and local:
Current	327	1,092	1,193
Deferred	(202)	(1,260)	50
Total U.S. state and local	125	(168)	1,243
Reconciliation of amounts computed by applying the U.S. federal income tax rate to pretax income before income taxes
Federal statutory rate (as a percent)	34.00%	34.00%	34.00%
State and local (as a percent)	1.10%	(0.70%)	16.70%
Stock options (as a percent)	(2.60%)	1.90%	4.00%
Non-controlling interests (as a percent)	(2.20%)	(2.00%)	(2.60%)
Valuation allowance (as a percent)			(172.90%)
Other (as a percent)	(3.30%)	3.80%	(2.60%)
Income taxes (as a percent)	27.00%	37.00%	(123.40%)
Foreign subsidiary deferred tax asset			318
Deferred tax assets:
Net operating loss carryforwards	1,917	3,735
Outside basis differences for U.S. partnerships	3,289	3,524
Stock options	2,086	1,397
Deferred revenue	873	1,014
Deferred compensation	340	407
State taxes	5
Other	347	455
Valuation allowance	(1,669)	(1,822)	(3,109)
Net deferred tax assets	7,188	8,710
Deferred tax liabilities:
Intangible assets	(1,539)	(1,202)
State taxes		(926)
Property and equipment	(189)	(133)
Net deferred tax liabilities	(1,728)	(2,261)
Net deferred taxes	5,460	6,449
Net operating loss carryforwards
Amount of net operating loss carryforwards	5,146
Release to the valuation allowance	51		400
Period of cumulative results for determination of releasing valuation allowance	3 years
Changes in the valuation allowance
Balance at the beginning of the period	1,822	3,109	14,989
Additions charged to operations	51		400
Decrease credited to operations	(204)	(1,287)	(12,280)
Balance at the end of the period	1,669	1,822	3,109
Windfall tax benefits from stock option exercises	2,190	246
Federal
Net operating loss carryforwards
Amount of net operating loss carryforwards	15,283	7,979
Release to the valuation allowance			12,280
Changes in the valuation allowance
Additions charged to operations			12,280
State
Net operating loss carryforwards
Amount of net operating loss carryforwards	29,955	27,900
Release to the valuation allowance		1,287
Changes in the valuation allowance
Additions charged to operations		1,287
Foreign
Net operating loss carryforwards
Amount of net operating loss carryforwards	$ 1,588	$ 1,334