Cash and Cash Equivalents (Additional Information) (Detail) - USD ($) $ in Millions	Mar. 31, 2017	Mar. 31, 2016
Cash and Cash Equivalents [Abstract]
Deposits at the Federal Reserve Bank earning interest at the Federal Funds target rate	$ 2,400	$ 1,100
Interest-earning deposits in other financial institutions	773	721
Fair value of securities purchased under agreements to resell	$ 182	$ 66

For banks and other depository institutions (including Federal Reserve Banks, if applicable): Interest-bearing deposits in other financial institutions for relatively short periods of time including, for example, certificates of deposits, which are presented separately from cash on the balance sheet.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.2)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=73530178&loc=d3e59706-112781

Name:	us-gaap_InterestBearingDepositsInBanks
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant