Allowance for Credit Losses - ECL Estimates (Details) $ in Millions	9 Months Ended
	Sep. 30, 2020 scenario	Jun. 30, 2020 scenario	Jan. 01, 2020 USD ($) scenario
Measurement of ECL Measurements [Line Items]
Number of forward-looking economic scenarios | scenario	4	4	3
Reasonable and supportable forecast period for consensus economic scenarios	2 years
Reversion period for average historical values	20 years
Reversion period for most assumption variables	3 years
Management judgment allowance for risk factors associated with large loan exposure in our commercial loan portfolio | $			$ 125
Downside Scenario [Member]
Measurement of ECL Measurements [Line Items]
Policy interest rate (percent, less than)			0.50%
Forecast Period 2020 [Member] | Central Scenario [Member]
Measurement of ECL Measurements [Line Items]
Inflation (percent)			2.00%
Forecast Period 2020 [Member] | Upside Scenario [Member]
Measurement of ECL Measurements [Line Items]
GDP growth rate (percent)			4.00%