Acquisitions and Dispositions (Merchant Solar Projects) (Narrative) (Detail) $ in Millions	1 Months Ended							3 Months Ended
	Dec. 31, 2017 USD ($) MW	Jun. 30, 2017 USD ($) MW	May 31, 2017 USD ($)	Apr. 30, 2017 MW	Mar. 31, 2017 USD ($) project	Feb. 28, 2017 USD ($)	Aug. 31, 2016 project	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Jan. 31, 2017	Sep. 30, 2016
Business Acquisition And Dispositions [Line Items]
Cost of project								$ 7	$ 94
Solar Projects from Cypress Creek Renewables, LLC
Business Acquisition And Dispositions [Line Items]
Percentage of equity interests acquired										100.00%
Cash consideration			$ 154
Cost of project		$ 160
MW capacity | MW		79
Community Energy Solar, LLC
Business Acquisition And Dispositions [Line Items]
Percentage of equity interests acquired											100.00%
Cash consideration						$ 29
Cost of project	$ 205
MW capacity | MW	100
Solar Frontier Americas Holding, LLC
Business Acquisition And Dispositions [Line Items]
Percentage of equity interests acquired							100.00%
Cash consideration					$ 77
Cost of project		$ 78
MW capacity | MW		30
Number of projects | project					1		2
Discontinued efforts on acquisition of additional MW capacity | MW				20