Revenue from Contracts with Customers - Contract Balances (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Disaggregation of Revenue [Line Items]
Contract Assets	$ 275		$ 278
Contract Liabilities	775		824
Change in contract assets included in other current assets	(3)
Change in contracts liabilities included in deferred revenue	(49)
Current portion of contract assets	205		$ 204
Amounts included in the beginning of year contract liability balance	$ 683	$ 528