Lease Obligations - Lessee - Lease Costs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Lease Cost [Line Items]
Total finance lease costs	$ 2	$ 2	$ 4	$ 3
Total operating lease costs	29	32	60	61
Total lease costs	31	34	64	64
Impairment losses and termination charges	3	7	5	7
Net occupancy and equipment expense
Lease Cost [Line Items]
Amortization of right-of-use assets	2	2	3	3
Interest on long-term debt
Lease Cost [Line Items]
Interest on lease liabilities	0	0	1	0
Net occupancy expense
Lease Cost [Line Items]
Operating lease cost	22	25	46	47
Short-term lease cost	0	0	1	0
Variable lease cost	7	8	14	16
Sublease income	$ 0	$ (1)	$ (1)	$ (2)