Employee Compensation and Benefit Plans - Pension Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Apr. 01, 2020
Defined Benefit Plan, Plan Assets, Allocation [Line Items]
Expected long-term rate of return on plan assets	5.00%
Actual rate of return on plan assets during period	21.00%
Expected long-term rate of return on investments	4.00%
Interest on projected benefit obligations	$ 52
Expected return on pension plan assets	(45)
Net pension expense	$ 7
Percent at quoted price	12.00%
Percent at similar assets	85.00%
Percent supported at unobservable inputs	3.00%
Postretirement benefit plan assets	$ 1,400	$ 1,200
Projected benefit obligations	2,300	2,100
Underfunded plan	$ 828	$ 892
Discount rate	3.00%
Contributions to benefit plan	$ 58
Expected future contributions	89
Expected payment, year one	97
Expected payment, years two and three	198
Expected payment, years four and five	(206)
Expected payment, thereafter	$ 548
Defined Benefit Plan, Equity Securities, US [Member]
Defined Benefit Plan, Plan Assets, Allocation [Line Items]
Target allocation, percentage	24.00%
Defined Benefit Plan, Equity Securities, Non-US [Member]
Defined Benefit Plan, Plan Assets, Allocation [Line Items]
Target allocation, percentage	17.00%
Fixed Income Securities [Member]
Defined Benefit Plan, Plan Assets, Allocation [Line Items]
Target allocation, percentage	44.00%
Defined Benefit Plan, Real Estate [Member]
Defined Benefit Plan, Plan Assets, Allocation [Line Items]
Target allocation, percentage	11.00%
Hedge Funds [Member]
Defined Benefit Plan, Plan Assets, Allocation [Line Items]
Target allocation, percentage	4.00%