Revenue from Contracts with Customers - Contract Assets Rollforward (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
Capitalized Cost To Fulfill Customer Contracts
Change in Capitalized Contract Costs
Balance at beginning of period	$ 216	$ 298
Additions	332	1,043
Amortization	(305)	(1,090)
Impairment	0	0
Foreign currency and other	5	(3)
Balance at end of period	248	248
Capitalized Cost To Obtain Customer Contracts
Change in Capitalized Contract Costs
Balance at beginning of period	144	145
Additions	16	37
Amortization	(9)	(30)
Impairment	0	0
Foreign currency and other	(1)	(2)
Balance at end of period	$ 150	$ 150