Lease Commitments - Lease Maturity (Details) $ in Millions	Mar. 31, 2019 USD ($)
Operating Lease Liabilities, Payments Due [Abstract]
Remainder of 2019	$ 206
2020	244
2021	217
2022	192
2023	138
Thereafter	494
Total undiscounted future minimum lease payments	1,491
Imputed interest	(161)
Present value of lease liabilities	1,330
Finance Lease Liabilities, Payments, Due [Abstract]
Remainder of 2019	26
2020	27
2021	23
2022	2
2023	0
Thereafter	0
Total undiscounted future minimum lease payments	78
Imputed interest	(1)
Present value of lease liabilities	77
Sublease Income, Payments Due [Abstract]
Remainder of 2019	(26)
2020	(31)
2021	(31)
2022	(32)
2023	(14)
Thereafter	4
Total undiscounted future minimum lease payments	(138)
Imputed interest	0
Present value of lease liabilities	(138)
Total
Remainder of 2019	206
2020	240
2021	209
2022	162
2023	124
Thereafter	490
Total undiscounted future minimum lease payments	1,431
Imputed interest	162
Present value of lease liabilities	$ 1,269