Employee Benefit Plans - MEC - Employer Contributions (Details) $ in Millions	9 Months Ended
	Sep. 30, 2021 USD ($)
Other Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, expected contributions in current fiscal year	$ 13
Employer contributions	10
MEC | Other Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, expected contributions in current fiscal year	12
Employer contributions	9
United States | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, expected contributions in current fiscal year	13
Employer contributions	9
United States | MEC | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, expected contributions in current fiscal year	7
Employer contributions	$ 5