Summary of Significant Accounting Policies and Nature of Operations Adoption of ASU 2016-13 Measurement of Credit Losses on Financial Instruments (Details) - USD ($)	3 Months Ended	9 Months Ended
	Mar. 31, 2020	Sep. 30, 2020
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	$ 147,000,000
Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Day one impact to allowance for credit losses	199,000,000
increase to deferred tax assets	52,000,000
Cumulative Effect of New Accounting Principle in Period of Adoption	$ 147,000,000
Initial economic forecast period		36 months
After initial economic forecast period, reverted straight-line basis, period		24 months
Impact from COVID-19 | Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Initial economic forecast period	24 months
Change in historical loss information		$ 0
Revised initial economic forecast period		24 months
Commercial | Impact from COVID-19
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loan Restructuring, Trial Modifications, Amount		$ 1,900,000,000
Consumer | Impact from COVID-19
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loan Restructuring, Trial Modifications, Amount		$ 2,400,000,000