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Accounting Changes - Credit Losses (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Accounting Changes
Deferred tax assets	$ 8,953	$ 9,241
Retained earnings	$ 162,218	$ 162,717
Accounting Standards Update 2016-13 \| Cumulative Effect, Period of Adoption, Adjustment
Accounting Changes
Allowance for credit losses			$ 81
Deferred tax assets			(14)
Retained earnings			$ (66)