Revenue from Contracts with Customers - Contract Assets Rollforward (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Capitalized Cost To Fulfill Customer Contracts
Change in Capitalized Contract Costs
Balance at beginning of period	$ 298
Additions	370
Amortization	(432)
Impairment	0
Foreign currency and other	4
Balance at end of period	240
Capitalized Cost To Obtain Customer Contracts
Change in Capitalized Contract Costs
Balance at beginning of period	145
Additions	8
Amortization	(10)
Impairment	0
Foreign currency and other	1
Balance at end of period	$ 144