Long-term debt (Details) (USD $)			1 Months Ended	12 Months Ended			1 Months Ended			1 Months Ended				1 Months Ended	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	May 31, 2011 New Credit Agreement	Dec. 31, 2011 New Credit Agreement quater	Dec. 31, 2010 New Credit Agreement	Aug. 31, 2010 New Credit Agreement	May 31, 2011 New Credit Agreement Maximum	Dec. 31, 2011 New Credit Agreement Base rate Maximum	Dec. 31, 2011 New Credit Agreement LIBOR Maximum	Aug. 31, 2010 Revolving Credit Facility Y	Dec. 31, 2011 Revolving Credit Facility	Dec. 31, 2011 Revolving Credit Facility Base rate	Dec. 31, 2011 Revolving Credit Facility LIBOR	Aug. 31, 2010 Term Loan Facility Y	Dec. 31, 2011 Term Loan Facility	Dec. 31, 2011 Term Loan Facility LIBOR
Long-term debt
Debt instrument term (in years)										5				5
Maximum borrowing capacity										$ 200,000,000				$ 100,000,000
Additional borrowing capacity						50,000,000
Amount outstanding	1,300,000	3,800,000									117,400,000				91,300,000
Margin on variable rate (as a percent)								2.25%	3.25%			2.00%	3.00%			3.00%
Preceding number of quarters in which consolidated leverage ratio is measured				4
Portion of credit facility charged at variable rate												17,400,000	100,000,000			100,000,000
Effective interest rate (as a percent)				3.40%	3.40%
Amount of restricted net assets	186,000,000	178,500,000
Restricted cash	47,105,000	22,944,000
Debt issuance costs incurred			800,000	5,000,000
Deferred debt issuance costs, net				3,500,000
Percentage of amortization of principal amount in year 1															5.00%
Percentage of amortization of principal amount in year 2															15.00%
Percentage of amortization of principal amount in year 3															25.00%
Percentage of amortization of principal amount in year 4															25.00%
Percentage of amortization of principal amount in year 5															30.00%
Aggregate maturities of long-term debt under contractual obligations
2012	17,500,000
2013	25,000,000
2014	26,300,000
2015	139,900,000
Specified settlement amounts excluded from EBITDA under amended agreement							$ 50,000,000