Variable Interest Entities - Net Assets of Texas Solar 4 (Details) - Texas Solar 4 - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Restricted cash	$ 5	$ 8
Non-utility property, less accumulated depreciation of $12 and $9, respectively	101	104
Other assets	8	43
Total assets	114	155
Long-term debt due within one year	2	3
Other liabilities	28	38
Long-term debt	58	60
Total liabilities	88	101
Accumulated depreciation	$ 12	$ 9