Summary of Significant Accounting Policies - Significant Accounting Policies (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Contract with Customer, Asset and Liability [Abstract]
Allowance for doubtful accounts for receivables
Contract assets
Receivables, net	898		898		885
Contract liabilities	50		50		$ 54
Revenue recognized that was included in the contract liability balance at the beginning of the reporting period
Revenue recognized from performance obligations satisfied in prior periods
Performance Guarantee [Member]
Loss Contingencies [Line Items]
Amounts accrued for performance guarantees
Amounts paid for performance guarantees