Pension and Other Postretirement Employee Benefit Plans - Schedule of Fair Values of Plan Assets (Details) - USD ($)	Jun. 30, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Expected contributions for 2019	$ 0
Pension plans
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	19,700,000,000	$ 18,100,000,000
OPEB plans
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 2,900,000,000	$ 2,600,000,000