Long-term Financing and Sales of Receivables - Credit Quality of Financing Receivables and Allowance for Credit Losses (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Long-term Receivable	$ 30	$ 37
Current Billed Due	1	1
Past Due Under 90 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	4
Past Due Over 90 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	3	3
Municipal leases secured tax exempt
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Long-term Receivable	17	21
Current Billed Due	0	0
Municipal leases secured tax exempt | Past Due Under 90 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	1
Municipal leases secured tax exempt | Past Due Over 90 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2	2
Commercial loans and leases secured
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Long-term Receivable	13	16
Current Billed Due	1	1
Commercial loans and leases secured | Past Due Under 90 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	3
Commercial loans and leases secured | Past Due Over 90 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 1	$ 1