Trading Securities (Detail) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities	[1]	$ 5,093	$ 6,067
Trading liabilities		1,283	1,351
US Treasury Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities		157	539
Trading liabilities		577	697
US Government Agencies Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities		395	480
US States and Political Subdivisions Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities		61	134
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities		700	567
Trading liabilities		0	1
Collateralized Loan Obligations [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities		0	1
Corporate Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities		655	656
Trading liabilities		289	255
Commercial Paper [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities		118	140
Equity Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities		56	49
Trading liabilities		9	0
Derivative Financial Instruments, Assets [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities	[2]	802	984
Loans [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities	[3]	2,149	2,517
Derivative Financial Instruments, Liabilities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading liabilities	[2]	$ 408	$ 398

[1] Includes trading securities pledged as collateral where counterparties have the right to sell or repledge the collateral of $1,086 million and $1,437 million at December 31, 2017 and December 31, 2016, respectively.

[2] Amounts include the impact of offsetting cash collateral received from and paid to the same derivative counterparties, and the impact of netting derivative assets and derivative liabilities when a legally enforceable master netting agreement or similar agreement exists.

[3] Includes loans related to TRS.