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Common and Preferred Stock (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
	Dec. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Nov. 01, 2019	Aug. 07, 2019
Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
AcceleratedShareRepurchaseProgramRepurchaseAmount						$ 150.0
Accelerated Share Repurchases, Cash or Stock Settlement	2,432,336
Common stock dividend rate percentage		5.00%
Common stock, par value		$ 5	$ 5
Antidilutive Securities Excluded from Computation of Earnings Per Share (in shares)		302,000	373,000	246,000
Shares available for purchase under the current Board authorization					5,000,000
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased		3,544,579

X

- Definition

Amount paid under the Accelerated Share Repurchase program.

+ References

No definition available.

+ Details

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Namespace Prefix:	cbsh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Common stock dividend rate percentage.

+ References

No definition available.

+ Details

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Namespace Prefix:	cbsh_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Description of the nature of the settlement (cash or stock) of the accelerated share repurchases agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 30
-Section 25
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6405686&loc=d3e22802-112653

+ Details

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X

- Definition

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

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Balance Type:	na
Period Type:	duration

X

- References

No definition available.

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X

- Definition

Face amount or stated value per share of common stock.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

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Balance Type:	na
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X

- Definition

The number of shares authorized to be repurchased by an entity's Board of Directors under a stock repurchase plan.

+ References

No definition available.

+ Details

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X

- Definition

The remaining number of shares authorized to be repurchased by an entity's Board of Directors under a stock repurchase plan.

+ References

No definition available.

+ Details

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