Leases - Lease Maturity (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Operating Leases, under ASC 842
Remainder of 2019	$ 48
2020	146
2021	129
2022	110
2023	95
Thereafter	254
Total lease payments	782
Less: Interest	(59)
Present value of lease liabilities	723
Finance Leases, under ASC 842
Remainder of 2019	16
2020	82
2021	70
2022	58
2023	7
Thereafter	6
Total lease payments	239
Less: Interest	(8)
Present value of lease liabilities	231
Operating lease payments related to options to extend lease terms that are reasonably certain of being exercised	55
Minimum operating lease payments for leases signed but not yet commenced	$ 6
Operating Leases, under ASC 840
2019		$ 94
2020		75
2021		62
2022		51
2023		40
Thereafter		108
Total		$ 430
Equipment | Minimum
Lessee, Lease, Description [Line Items]
Operating leases not yet commenced, term	1 year
Equipment | Maximum
Lessee, Lease, Description [Line Items]
Operating leases not yet commenced, term	10 years