Pension, Savings, and Other Employee Benefits - Projected Benefit Obligation Affected by Interest Crediting Ratings (Details) - Pension Benefits $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) pension_plan	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Number of plans affected by interest credit ratings | pension_plan	1
Defined benefit plan, benefit obligation | $	$ 15	$ 16	$ 17
Interest crediting rate	8.20%	9.66%	10.12%