Commitments - Standby and Commercial Letters of Credit (Parenthetical) (Details) - Financial Standby Letter of Credit $ in Millions	Sep. 30, 2019 USD ($)
Line of Credit Facility [Line Items]
Commitments	$ 2,219
Less Than One Year From The Balance Sheet Date
Line of Credit Facility [Line Items]
Commitments	1,124
Less Than One Year From The Balance Sheet Date | Commercial
Line of Credit Facility [Line Items]
Commitments	1
More than One and within Five Years from Balance Sheet Date
Line of Credit Facility [Line Items]
Commitments	1,056
More than One and within Five Years from Balance Sheet Date | Commercial
Line of Credit Facility [Line Items]
Commitments	5
More than Five Years from Balance Sheet Date
Line of Credit Facility [Line Items]
Commitments	$ 39