Benefit Plans - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Number of Highest Consecutive Years of Earnings			5 years
Defined benefit plan, final years of employment subject to earnings test			10 years
Expected long-term rate of return on plan assets			7.00%	7.00%	7.00%
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent			50.00%
Defined Contribution Plan, Minimum Age of Employees Covered			21 years
Defined Contribution Plan, Minimum Number of Years of Service of Employees Covered			1 year
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Compensation, Maximum			6.00%
Defined Contribution Plan, Employer Contribution Expense			$ 141	$ 133	$ 129
Fair value of plan assets			5,669	6,152
Alternative Investments [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets			$ 274	$ 105
Subsequent Event
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discretionary contributions	$ 549
Forecast
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected long-term rate of return on plan assets		7.00%