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Long-term debt - Additional Information (Detail) (USD $)
 1 Months Ended 9 Months Ended 1 Months Ended 9 Months Ended 12 Months Ended 1 Months Ended 12 Months Ended 9 Months Ended
May 31, 2012
Sep. 30, 2012
Sep. 30, 2011
Dec. 31, 2011
Sep. 30, 2012
Revolving Credit Facility [Member]
May 31, 2012
Revolving Credit Facility [Member]
Aug. 31, 2010
Revolving Credit Facility [Member]
Sep. 30, 2012
Revolving Credit Facility [Member]
Dec. 31, 2011
Revolving Credit Facility [Member]
Jun. 30, 2012
Revolving Credit Facility [Member]
Aug. 30, 2010
Revolving Credit Facility [Member]
Sep. 30, 2012
Revolving Credit Facility [Member]
LIBOR [Member]
Jun. 30, 2012
Revolving Credit Facility [Member]
LIBOR [Member]
Dec. 31, 2011
Revolving Credit Facility [Member]
LIBOR [Member]
Dec. 31, 2011
Revolving Credit Facility [Member]
Base rate [Member]
May 31, 2012
Term Loan Facility [Member]
Aug. 31, 2010
Term Loan Facility [Member]
Dec. 31, 2011
Term Loan Facility [Member]
Sep. 30, 2012
New Credit Agreement [Member]
Quarters
Dec. 31, 2011
New Credit Agreement [Member]
Aug. 31, 2010
New Credit Agreement [Member]
Sep. 30, 2012
New Credit Agreement [Member]
LIBOR [Member]
Maximum [Member]
Sep. 30, 2012
New Credit Agreement [Member]
Base rate [Member]
Maximum [Member]
Debt Instrument [Line Items]                                              
Debt instrument term (in years)             5 years                   5 years            
Maximum borrowing capacity                     $ 200,000,000                        
Maximum borrowing capacity                                 100,000,000            
Maximum additional borrowing capacity available                                         50,000,000    
Repayment of debt obligation   188,695,000 3,750,000                         87,500,000              
Repayment of debt obligation         20,000,000 57,500,000                                  
Amount outstanding   300,000   1,300,000 20,000,000     20,000,000 117,400,000 20,000,000                          
Amount outstanding   20,000,000   191,195,000                           91,300,000          
Variable rate basis               LIBOR LIBOR                 LIBOR          
Margin on variable rate (as a percent)                       2.50% 2.50% 3.00% 2.00%             3.25% 2.25%
Preceding number of quarters in which consolidated leverage ratio is measured                                     4        
Portion of credit facility charged at variable rate                           100,000,000 17,400,000                
Effective interest rate (as a percent)                                     2.74% 3.40%      
Amount of restricted net assets   204,800,000   186,000,000                                      
Restricted cash   67,319,000   45,476,000                                      
Debt issuance costs incurred   5,000,000                                          
Write-off of debt issue costs 800,000                                            
Deferred debt issuance costs, net   $ 2,000,000   $ 3,500,000