Summary of Significant Accounting Policies - Significant Accounting Policies (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Contract with Customer, Asset and Liability [Abstract]
Allowance for doubtful accounts for receivables
Contract assets
Receivables, net	911		911		885
Contract liabilities	56		56		$ 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