Annual report pursuant to Section 13 and 15(d)

Operating Revenues under Customer Agreements - Additional Information (Details)

v3.8.0.1
Operating Revenues under Customer Agreements - Additional Information (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Leases, Operating [Abstract]      
Contingent revenue from customers $ 109.8 $ 86.2 $ 59.8