Summary of significant accounting policies (Details 5) (USD $)	0 Months Ended	12 Months Ended
	May 03, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 CCDG	Dec. 31, 2011 CCDG	Dec. 31, 2010 CCDG	Dec. 31, 2009 CCDG
Non-controlling interests
Percentage of net profits less capital expenditures of the preceding year allocated to non-controlling interest holders					30.00%
Distributions to non-controlling interest holders		$ 678,000	$ 547,000	$ 398,000
Initial term of Detroit Operating Agreement					7 years
Optional extension period of Detroit Operating Agreement					4 years
Amount of annual fixed distribution per year under Detroit Operating Agreement					121,000	85,000	85,000
Non-controlling owners capital account amount		$ 823,000	$ 197,000					$ 0
Convertible preferred stock
Preferred stock issued (in shares)		0	0
Preferred stock outstanding (in shares)		0	0
Warrants exercisable into convertible preferred stock
Preferred stock warrants outstanding (in shares)	25,196
Issuance of common stock upon exercise and conversion of preferred stock warrants (in shares)	20,172