Benefit Obligations (Pension and Other Postretirement Obligations Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Pension and postretirement net periodic benefit cost	$ (80)	$ (54)	$ (52)
Multiemployer plans, funded status	At least 80 percent	At least 80 percent	At least 80 percent
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Settlement gain (loss)	$ 0	$ 0	$ 0
Curtailment gain (loss)	0	0	3
Amortization of prior service cost (credit)	0	0	0
Lump-sum buyout payments	1	1
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Settlement gain (loss)	0	0	0
Curtailment gain (loss)	0	0	0
Amortization of prior service cost (credit)	(1)	(3)	$ 0
Lump-sum buyout payments	$ 0	0
Change in Assumptions for Defined Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and postretirement net periodic benefit cost		$ 29