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Summary of Significant Accounting Policies - Additional Information (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Accounting Policies [Abstract]        
Motor fuel and sales taxes $ 306.8 $ 303.7 $ 888.3 $ 878.0