Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	12 Months Ended
	Dec. 31, 2018	Jun. 30, 2018	Jun. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Curtailments			$ 9
Pension benefits paid
Defined Benefit Plan Disclosure [Line Items]
Service cost				$ 25	$ 46	$ 90
Interest cost				235	219	234
Expected return on plan assets				(286)	(306)	(332)
Recognized net actuarial loss				139	168	220
Amortization of prior service cost (benefit)				2	3	5
Settlements				9	96	0
Curtailments				0	23	0
Net periodic benefit cost				124	249	217
Pension benefits paid | United States
Defined Benefit Plan Disclosure [Line Items]
Curtailments		$ 5		17
Net periodic benefit cost				127	239	206
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost				7	7	7
Interest cost				28	28	30
Expected return on plan assets				0	0	0
Recognized net actuarial loss				4	7	5
Amortization of prior service cost (benefit)				(6)	(7)	(8)
Settlements				0	0	0
Curtailments	$ 9			(58)	(28)	0
Net periodic benefit cost				(25)	7	34
Decrease in net periodic benefit cost for the recognition of the federal subsidy awarded under Medicare Part D				$ 11	$ 10	$ 11