Auction rate securities held at fair value

No definition available.

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

- Definition

The value of common stock held by the holding company.

+ References

No definition available.

+ Details

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

- Definition

Fair value of securities on other-than-temporary impairment watch list.

+ References

No definition available.

+ Details

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

- Definition

The fair value of investment securities pledged for public funds, repurchase agreements, and borrowings

+ References

No definition available.

+ Details

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

- Definition

The number of investments in a single issuer that exceed 10% of stockholder's equity, excluding obligations of various government-sponsored enterprises such as FNMA, FHLB, and FHLMC.

+ References

No definition available.

+ Details

Name:	cbsh_InvestmentsExceedingTenPercentOfStockholdersEquity
Namespace Prefix:	cbsh_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Non-marketable private equity investment securities held

+ References

No definition available.

+ Details

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

- Definition

The carrying amount of Federal Home Bank and Federal Reserve Board stock owned. These securities are not readily marketable but may be pledged for debt or other regulatory requirements

+ References

No definition available.

+ Details

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

- Definition

State and municipal obligations, excluding auction rate securities, as a percent of total available for sale securities

+ References

No definition available.

+ Details

Name:	cbsh_PercentOfStateAndMunicipalObligationPortfolio
Namespace Prefix:	cbsh_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The percent of decrease in the fair value of securities below amortized cost requiring a review for possible other-than-temporary impairment.

+ References

No definition available.

+ Details

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

- Definition

No investment in a single issuer exceeds this percentage of stockholder's equity, other than obligations of various government-sponsored enterprises such as FNMA, FHLB, and FHLMC.

+ References

No definition available.

+ Details

Name:	cbsh_PercentageOfStockholdersEquityOfWhichNoInvestmentFromSingleIssuerExceeds
Namespace Prefix:	cbsh_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

U.S. Treasury inflation protected securities held at fair value

+ References

No definition available.

+ Details

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

- Definition

Gross unrealized losses on auction rate securities

+ References

No definition available.

+ Details

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

- Definition

Line items represent reporting 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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 12
-Sentence Column A

+ Details

Name:	invest_InvestmentLineItems
Namespace Prefix:	invest_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562

+ Details

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

- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27290-111563

+ Details

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

- Definition

Fair value of financial instrument held by the entity for its own account (proprietary securities) for trading or investment purposes that are carried at fair value and pledged to counterparties as collateral for financing transactions where counterparty has the right to re-pledge such.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66007379&loc=d3e113888-111728

+ Details

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

- Definition

The cumulative amount of credit losses recognized in earnings related to debt securities held for which a portion of an other than temporary impairment (OTTI) was recognized in other comprehensive income (a component of shareholders' equity).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 8B
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=SL6284393-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 8B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=SL6284393-111563

+ Details

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

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=cbsh_NonAgencyMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Investment Securities (Narrative) (Details) $ in Thousands	9 Months Ended
Investment Securities (Narrative) (Details) $ in Thousands	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)
Investment [Line Items]
Non-marketable FHLB and FRB stock held for debt and regulatory purposes	$ 47,300	$ 46,900
Non-marketable private equity investments held	51,700	52,300
U.S. Treasury inflation-protected securities held, at fair value	$ 438,700
Percentage decrease requiring a review for impairment	20.00%
Fair value of securities on other-than-temporary impairment watch list	$ 63,600	79,600
Credit-related portion of impairments included in earnings, for securities on watch list	14,245	14,080	$ 14,228	$ 14,129
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	28,352	60,942
Available for sale securities	9,109,287	9,649,203
Securities pledged as collateral	3,700,000
Securities pledged as collateral and may be sold or re-pledged	$ 667,200
Number of investments in a single issuer that exceed 10% of stockholder's equity	0
No investment in a single issuer exceeds this percentage of stockholder's equity	10.00%
State And Municipal Obligations [Member]
Investment [Line Items]
Auction rate securities held at fair value	$ 16,900
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	1,685	12,693
Unrealized Losses on Auction Rate Securities	221
Available for sale securities	$ 1,720,403	1,778,214
State and municipal obligation portfolio, excluding auction rate securities, as a percentage of total available for sale securities	18.90%
Equity Securities [Member]
Investment [Line Items]
Common stock held by the holding company	$ 83,400
Available for sale securities	83,522	51,263
Non-Agency Mortgage-Backed Securities [Member]
Investment [Line Items]
Fair value of securities on other-than-temporary impairment watch list	28,100
Credit-related portion of impairments included in earnings, for securities on watch list	14,200
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	3,851	6,493
Available for sale securities	$ 1,066,764	$ 1,055,639