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Subsequent Events - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Millions
 1 Months Ended
Oct. 11, 2017
Oct. 31, 2017
Sep. 30, 2017
Sep. 30, 2016
Subsequent Event [Line Items]        
Letters of credit issued and outstanding     $ 48.0 $ 50.6
Subsequent Event        
Subsequent Event [Line Items]        
Cash deposit in support of liability $ 34.2      
Decrease in outstanding letters of credit 36.6      
Letters of credit issued and outstanding $ 11.4      
Subsequent Event | Dividend Declared        
Subsequent Event [Line Items]        
Distribution declared   $ 0.11    
Partners capital projected distribution amount on annualized basis   0.44    
Minimum dividend distribution per unit   $ 0.0675    
Percentage of distributions to common unit holders in excess of minimum quarterly distribution   90.00%    
Percentage of distributions to general unit holders in excess of minimum quarterly distribution   10.00%    
Amount to paid to common unit holders   $ 6.1    
Amount to paid to the General Partner   0.2    
Incentive distribution to the General Partner   0.1    
Incentive distributions to management   $ 0.1    
Dividend payable date   Oct. 31, 2017    
Dividend record date   Oct. 23, 2017