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Commitments and Contingencies - Outstanding Capital Commitments Associated with Investments in Unconsolidated Affiliates (Parenthetical) (Detail)
$ in Millions
1 Months Ended
Aug. 31, 2017
USD ($)
Other Commitments, Engineering, Procurement and Construction Electric Transmission Project [Member] | Corporate Joint Venture [Member] | Scenario Forecast [Member]  
Other Commitments [Line Items]  
Return of capital from unconsolidated affiliates anticipated for 2017 $ 42.1