Employee Benefit Plans - Narrative (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in post-retirement benefit liability			$ (102,000,000)
Employer contributions			0	$ 0	$ 250,000,000
Expected future contributions			0
Pension plan assets at fair value			2,431,000,000	2,833,000,000
Special termination benefits			56,000,000
Amount of premium expenses incurred under the plan			30,000,000	40,000,000	35,000,000
Expense associated with savings plans			$ 41,000,000	39,000,000	35,000,000
Equity
Defined Benefit Plan Disclosure [Line Items]
Target allocation percentage of pension assets			70.00%
Targeted allocation of total plan assets in domestic equity			42.00%
Targeted allocation of total plan assets in international equity			28.00%
Percentage within which the planned allocation is managed			3.00%
Pension plan assets at fair value			$ 1,698,000,000	2,060,000,000
Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Target allocation percentage of pension assets			30.00%
Pension plan assets at fair value			$ 704,000,000	729,000,000
Pension and Other Post-Employment Benefits
Defined Benefit Plan Disclosure [Line Items]
Gain (loss) recognized in other comprehensive income			102,000,000
Post-retirement benefits
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in post-retirement benefit liability			(102,000,000)	13,000,000
Pension plan assets at fair value			0	0	0
Special termination benefits			0	13,000,000	0
Curtailment loss				13,000,000
Pre-tax gain (loss) to be amortized			103,000,000
Post-retirement benefits | Nonqualified Plan
Defined Benefit Plan Disclosure [Line Items]
Employer contributions			30,000,000	29,000,000
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in post-retirement benefit liability			0	58,000,000
Pension plan assets at fair value			2,431,000,000	2,833,000,000	2,539,000,000
Decrease to pension liabilities			85,000,000
Special termination benefits			0	60,000,000	$ 0
Curtailment loss				4,000,000
Pre-tax gain (loss) to be amortized			(888,000,000)
Pension Plans | Qualified Plan
Defined Benefit Plan Disclosure [Line Items]
Settlement loss				11,000,000
Pension Plans | Nonqualified Plan
Defined Benefit Plan Disclosure [Line Items]
Employer contributions			$ 17,000,000	45,000,000
Settlement loss				$ 12,000,000
Subsequent Event | Plan | Post-retirement benefits
Defined Benefit Plan Disclosure [Line Items]
Life insurance and contributions to the medical program for eligible retirees will be eliminated (younger than)		65 years
Life insurance and contributions to the medical program for eligible retirees will be eliminated (65 or older)	65 years