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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	$ 42.5

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Cash dividends or other distributions received from unconsolidated subsidiaries, certain corporate joint ventures, and certain noncontrolled corporations that are returns of capital. Excludes dividends or distributions from equity method investments classified as operating activities.

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