Acquisitions and Divestitures - Additional Information (Detail) $ in Millions		1 Months Ended	3 Months Ended		9 Months Ended	12 Months Ended
	Nov. 01, 2016 USD ($)	Feb. 28, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($) Employees HydroelectricPowerDevelopments
Business Disposition [Line Items]
Cash provided at separation to Parent Company	$ 1,072				$ 247
Other current liabilities					376	$ 707
Yadkin [Member]
Business Disposition [Line Items]
Net cash received		$ 246	$ 5	$ 241
Income (loss) from divestitures before income tax					120
Income (loss) from divestitures after income tax					120
Cash provided at separation to Parent Company					$ 243	$ 243
Number of hydroelectric power developments | HydroelectricPowerDevelopments						4
Sale generated						$ 29
Number of employees | Employees						30
Other current liabilities						$ 243