Shareholders' Equity - Summary of Amount of Common Stock Repurchased by the Firm (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2020
Equity [Abstract]
Common share repurchases	0.0	8.2
Average cost per share	$ 0	$ 236.35
Total cost of common share repurchases	$ 0	$ 1,928

Equity impact of the cost of common and preferred stock that were repurchased during the period. Recorded using the cost method.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration