Other Operating Expenses, Net (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Other Operating Expenses, Net [Abstract]
Merger and restructuring costs	$ 28	$ 131	$ 76	$ 226
Special charges, net	7	4	35	6
Gain on sale of assets, net	(6)	0	(13)	(3)
Other operating expenses, net	29	135	98	229
Merger And Restructuring Costs [Roll Forward]
Accrued merger and restructuring costs, beginning of period			198	276	$ 276
Costs incurred			71		302
Cash paid			(144)		(380)
Accrued merger and restructuring costs, end of period	125		125		198
Stock compensation expense recognized in merger costs		$ 20	5	37
Obligation charge arising from a withdrawal liability from a multi-employer pension plan			22
Employee Retention Costs [Member]
Merger And Restructuring Costs [Roll Forward]
Accrued merger and restructuring costs, beginning of period			1	7	7
Costs incurred			0		4
Cash paid			0		(10)
Accrued merger and restructuring costs, end of period	1		1		1
Employee Termination Costs [Member]
Merger And Restructuring Costs [Roll Forward]
Accrued merger and restructuring costs, beginning of period			172	244	244
Costs incurred			49		226
Cash paid			(128)		(298)
Accrued merger and restructuring costs, end of period	93		93		172
Transaction and Advisory Costs [Member]
Merger And Restructuring Costs [Roll Forward]
Accrued merger and restructuring costs, beginning of period			17	25	25
Costs incurred			1		4
Cash paid			0		(12)
Accrued merger and restructuring costs, end of period	18		18		17
Other Costs [Member]
Merger And Restructuring Costs [Roll Forward]
Accrued merger and restructuring costs, beginning of period			8	$ 0	0
Costs incurred			21		68
Cash paid			(16)		(60)
Accrued merger and restructuring costs, end of period	$ 13		$ 13		$ 8