Lease Commitments - Lease Maturity (Details) $ in Millions	Sep. 30, 2019 USD ($)
Lessee, Operating Lease, Liability, Payment, Due [Abstract]
Remainder of 2019	$ 61
2020	246
2021	225
2022	199
2023	144
Thereafter	513
Total undiscounted future minimum lease payments	1,388
Imputed interest	(143)
Present value of lease liabilities	1,245
Finance Lease, Liability, Payment, Due [Abstract]
Remainder of 2019	7
2020	26
2021	26
2022	19
2023	18
Thereafter	15
Total undiscounted future minimum lease payments	111
Imputed interest	(5)
Present value of lease liabilities	106
Rentals from subleases
Remainder of 2019	(9)
2020	(34)
2021	(33)
2022	(33)
2023	(14)
Thereafter	(6)
Total undiscounted future minimum lease payments	(129)
Imputed interest	0
Present value of lease liabilities	(129)
Total
Remainder of 2019	59
2020	238
2021	218
2022	185
2023	148
Thereafter	522
Total undiscounted future minimum lease payments	1,370
Imputed interest	(148)
Present value of lease liabilities	$ 1,222