Employee Retirement Benefits - Net Periodic Income Cost Regulatory Assets (Details) - Pension Benefits [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Unrecognized losses (gains)	$ (113)	$ 216
Amortization of prior service cost	1	1
Total	$ (112)	$ 217