Activity in the ALLL by segment (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Allowance for Loan and Lease Losses [Roll Forward]
Provision for loan losses	$ 38	$ 117
Allowance for Loan and Lease Losses, Write-offs	(106)	(146)
Loan recoveries	27	34
Loans and Leases Receivable, Allowance	1,694	1,714	$ 1,735	$ 1,709
Commercial Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Provision for loan losses	(16)	46
Allowance for Loan and Lease Losses, Write-offs	(23)	(63)
Loan recoveries	6	13
Loans and Leases Receivable, Allowance	1,068	1,120	1,101	1,124
Consumer Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Provision for loan losses	54	71
Allowance for Loan and Lease Losses, Write-offs	(83)	(83)
Loan recoveries	21	21
Loans and Leases Receivable, Allowance	$ 626	$ 594	$ 634	$ 585

Amount of recovery of loans and lease receivables which had previously been fully or partially written-off as bad debts.

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

+ References

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- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
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-Publisher FASB
-Name Accounting Standards Codification
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-Section S99
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- Definition

Amount of expense related to estimated loss from loan and lease transactions.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
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