Lease Commitments - Lease Maturity (Details) $ in Millions	Jun. 30, 2019 USD ($)
Lessee, Operating Lease, Liability, Payment, Due [Abstract]
Remainder of 2019	$ 127
2020	248
2021	224
2022	198
2023	144
Thereafter	511
Total undiscounted future minimum lease payments	1,452
Imputed interest	(151)
Present value of lease liabilities	1,301
Finance Lease, Liability, Payment, Due [Abstract]
Remainder of 2019	14
2020	28
2021	22
2022	2
2023	0
Thereafter	0
Total undiscounted future minimum lease payments	66
Imputed interest	(1)
Present value of lease liabilities	65
Rentals from subleases
Remainder of 2019	(17)
2020	(34)
2021	(33)
2022	(34)
2023	(15)
Thereafter	(8)
Total undiscounted future minimum lease payments	(141)
Imputed interest	0
Present value of lease liabilities	(141)
Total
Remainder of 2019	124
2020	242
2021	213
2022	166
2023	129
Thereafter	503
Total undiscounted future minimum lease payments	1,377
Imputed interest	(152)
Present value of lease liabilities	$ 1,225