Quarterly report pursuant to Section 13 or 15(d)

Commitments and Contingencies - Investments in Affiliates and Other Entities (Detail)

v3.7.0.1
Commitments and Contingencies - Investments in Affiliates and Other Entities (Detail)
$ in Millions
 3 Months Ended 33 Months Ended
Dec. 31, 2017
USD ($)
Jun. 30, 2017
USD ($)
May 31, 2022
USD ($)
Sep. 30, 2017
USD ($)
Dec. 31, 2014
kV
substation
km
Other Commitments, Planned Oil and Gas Infrastructure Projects          
Other Commitments [Line Items]          
Outstanding capital commitment   $ 18.1      
Capital for infrastructure projects          
Other Commitments [Line Items]          
Outstanding capital commitment   60.0      
Scenario, Forecast | Other Commitments, Planned Oil and Gas Infrastructure Projects          
Other Commitments [Line Items]          
Outstanding capital commitment     $ 17.7 $ 0.4  
EPC Electric Transmission Project          
Other Commitments [Line Items]          
Length of electrical transmission line to be constructed under contract | km         500
Number of substations | substation         2
Voltage of substations | kV         500
Aggregate contributions to this unconsolidated affiliate   26.7      
Proceeds from return of capital   3.0      
Corporate Joint Venture | Scenario, Forecast | Other Commitments, Planned Oil and Gas Infrastructure Projects          
Other Commitments [Line Items]          
Proceeds from return of capital $ 56.3        
Infrastructure investors partnership | Capital for infrastructure projects          
Other Commitments [Line Items]          
Outstanding capital commitment   $ 750.0