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	6 Months Ended
	Mar. 31, 2020	Jun. 30, 2020	Jan. 01, 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]
Deferred tax asset			$ 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
Accounting Standards Update 2016-13 | Cumulative Effect, Period Of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			$ 199,000,000
Impact on Operations 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 on Operations from COVID-19
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loan restructuring, modifications, amount		$ 2,700,000,000
Consumer | Impact on Operations from COVID-19
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loan restructuring, modifications, amount		$ 3,800,000,000