The ratio of Allowance for Credit Losses to All Loans.

No definition available.

Name:	cma_FinancingReceivableRatioOfAllowanceForCreditLossesToAllLoans
Namespace Prefix:	cma_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount, after recoveries, of loans and leases that have been written-off.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

A valuation allowance for financing receivables that are expected to be uncollectible.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of recovery of financing receivables doubtful of collection that were previously charged off.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of direct write-downs of financing receivables charged against the allowance.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

+ Details

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

- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ References

+ Details

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

- Definition

Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5074-111524

+ Details

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

- Definition

Amount of expense related loan transactions, lease transactions, credit loss from transactions other than loan and lease transactions, and other loss based on assessment of uncollectability from the counterparty to reduce the account to their net realizable value.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.11)
-URI http://asc.fasb.org/extlink&oid=114873765&loc=SL114874048-224260

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

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- Details

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- Details

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Period Type:

Credit Quality And Allowance For Credit Losses (Changes In The Allowance For Loan Losses And Related Loan Amounts) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Financing Receivable, Allowance for Credit Losses [Line Items]
Provision for loan losses	$ (13)	$ 12
Total loans			$ 50,302	$ 50,163
Financing Receivable
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period	671	712
Loan charge-offs	(20)	(37)
Recoveries on loans previously charged-off	9	9
Net loan recoveries (charge-offs)	(11)	(28)
Provision for loan losses	(13)	14
Balance at end of period	647	698
As a percentage of total loans			1.29%		1.42%
Allowance for loan losses individually evaluated for impairment			$ 35		$ 42
Allowance for loan losses collectively evaluated for impairment			612		656
Total allowance for loan losses	671	712	647	671	698
Loans individually evaluated for impairment			236		415
Loans collectively evaluated for impairment			50,066		48,825
Total loans			$ 50,302		$ 49,240
Business loans \| Financing Receivable
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period	627	661
Loan charge-offs	(19)	(36)
Recoveries on loans previously charged-off	8	8
Net loan recoveries (charge-offs)	(11)	(28)
Provision for loan losses	(8)	20
Balance at end of period	608	653
As a percentage of total loans			1.33%		1.46%
Allowance for loan losses individually evaluated for impairment			$ 35		$ 42
Allowance for loan losses collectively evaluated for impairment			573		611
Total allowance for loan losses	627	661	608	627	653
Loans individually evaluated for impairment			202		384
Loans collectively evaluated for impairment			45,660		44,339
Total loans			45,862		$ 44,723
Retail loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans			$ 4,440	4,484
Retail loans \| Financing Receivable
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period	44	51
Loan charge-offs	(1)	(1)
Recoveries on loans previously charged-off	1	1
Net loan recoveries (charge-offs)	0	0
Provision for loan losses	(5)	(6)
Balance at end of period	39	45
As a percentage of total loans			0.88%		0.99%
Allowance for loan losses individually evaluated for impairment			$ 0		$ 0
Allowance for loan losses collectively evaluated for impairment			39		45
Total allowance for loan losses	$ 44	$ 51	39	$ 44	45
Loans individually evaluated for impairment			34		31
Loans collectively evaluated for impairment			4,406		4,486
Total loans			$ 4,440		$ 4,517