Securities - Narrative (Details) - USD ($) $ in Millions			9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2020
Debt Securities, Available-for-sale [Line Items]
Accounting Standards Update [Extensible List]			us-gaap:AccountingStandardsUpdate201602Member	us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201802Member
Debt securities, available-for-sale, allowance for credit loss	$ 12
AOCI, transfers from AFS to HTM Securities, gross unrealized losses	$ 49	$ 65
Cumulative Effect, Period Of Adoption, Adjustment
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, allowance for credit loss						$ 7