Basis of Presentation and Summary of Significant Accounting Policies - Components of Restricted Cash (Details) - USD ($) $ in Millions	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Accounting Policies [Line Items]
Current		$ 225	$ 299
Non-current		16	46
Total		241	345
Debt service
Accounting Policies [Line Items]
Current		55	58
Non-current		8	8
Total		63	66
Construction major maintenance
Accounting Policies [Line Items]
Current		35	28
Non-current		7	6
Total		42	34
Margin Deposits Received/Returned to PG&E [Member]
Accounting Policies [Line Items]
Total		119
Security project insurance
Accounting Policies [Line Items]
Current		130	209
Non-current		0	31
Total		130	240
Other
Accounting Policies [Line Items]
Current		5	4
Non-current		1	1
Total		$ 6	$ 5
Subsequent Event | Debt service
Accounting Policies [Line Items]
Total	$ 52
Subsequent Event | Construction major maintenance
Accounting Policies [Line Items]
Total	2
Subsequent Event | Security project insurance
Accounting Policies [Line Items]
Total	$ 34