Benefit Obligations (Pension and Other Postretirement Obligations Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Multiemployer Plan, Pension, Insignificant, Funded Status [Fixed List]	At least 80 percent	At least 80 percent	At least 80 percent
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Curtailment gain (loss)	$ 0	$ 0	$ 1
Lump-sum buyout payments	7	3
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Curtailment gain (loss)	1	0	$ 0
Lump-sum buyout payments	$ 0	$ 0