Employee Pension and Other Postretirement Benefits - Executive Supplemental Benefit Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Plan and Other Postretirement Benefits Plan
Employee pre-tax covered compensation percent	75.00%
Employee after-tax covered compensation percent	10.00%
Combined maximum employee contribution percent	75.00%
Employee pre-tax gross pay for which the employer contributes a match	4.00%
Contribution plan expenses	$ 61	$ 59	$ 55
Executive Supplemental Benefits Pension Plan
Pension Plan and Other Postretirement Benefits Plan
Accrued liability for ESBPs	$ 94	$ 89
Matching Range 1
Pension Plan and Other Postretirement Benefits Plan
Company pre-tax compensation match	100.00%
Employee pre-tax gross pay for which the employer contributes a match	3.00%
Matching Range 2
Pension Plan and Other Postretirement Benefits Plan
Company pre-tax compensation match	50.00%
Employee pre-tax gross pay for which the employer contributes a match	2.00%