Lease Obligations - Lessee - Lease Assets and Lease Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Leases [Abstract]
Operating lease right-of-use assets	$ 436	$ 423
Operating lease right-of-use assets [Extensible List]	Other assets	Other assets
Finance lease right-of-use assets	$ 144	$ 129
Finance lease, right-of-use assets [Extensible List]	Bank premises and equipment	Bank premises and equipment
Total right-of-use assets	$ 580	$ 552
Operating lease liabilities	$ 531	$ 527
Operating lease liabilities [Extensible List]	Accrued Liabilities [Member]	Accrued Liabilities [Member]
Finance lease liabilities	$ 147	$ 130
Finance lease liabilities [Extensible List]	Long-term debt	Long-term debt
Total lease liabilities	$ 678	$ 657
Operating lease right of use asset, accumulated amortization	182	152
Finance lease right of use asset, accumulated amortization	$ 42	$ 29