Leases - Lease Maturity (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Operating Leases, under ASC 842
2019	$ 69
2020	77
2021	65
2022	53
2023	42
Thereafter	123
Total lease payments	429
Less: Interest	(47)
Present value of lease liabilities	382
Finance Leases, under ASC 842
2019	2
2020	5
2021	6
2022	0
2023	0
Thereafter	0
Total lease payments	13
Less: Interest	0
Present value of lease liabilities	13
Operating lease payments related to options to extend lease terms that are reasonably certain of being exercised	32
Minimum lease payments for leases signed but not yet commenced	$ 8
Operating Leases, under ASC 840
2019		$ 94
2020		75
2021		62
2022		51
2023		40
Thereafter		108
Total		$ 430
Minimum
Lessee, Lease, Description [Line Items]
Leases not yet commenced, term	3 years
Equipment | Maximum
Lessee, Lease, Description [Line Items]
Leases not yet commenced, term	7 years