Investment Securities (Available For Sale Securities Unrealized Gains And Losses, By Security Type) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Investment [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	$ 8,496,993	$ 8,602,599
Available For Sale Securities, Gross Unreailzed Gain at Period End	169,967	35,266
Available For Sale Securities, Gross Unrealized Loss at Period End	(6,541)	(99,824)
Debt Securities, Available-for-sale, Fair Value	8,660,419	8,538,041
U.S. Government and Federal Agency Obligations [Member]
Investment [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	826,551	914,486
Available For Sale Securities, Gross Unreailzed Gain at Period End	23,878	4,545
Available For Sale Securities, Gross Unrealized Loss at Period End	(552)	(11,379)
Debt Securities, Available-for-sale, Fair Value	849,877	907,652
Government-Sponsored Enterprise Obligations [Member]
Investment [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	167,683	199,470
Available For Sale Securities, Gross Unreailzed Gain at Period End	1,172	55
Available For Sale Securities, Gross Unrealized Loss at Period End	(38)	(3,747)
Debt Securities, Available-for-sale, Fair Value	168,817	195,778
State and Municipal Obligations [Member]
Investment [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	1,163,411	1,322,785
Available For Sale Securities, Gross Unreailzed Gain at Period End	40,097	10,284
Available For Sale Securities, Gross Unrealized Loss at Period End	(13)	(5,030)
Debt Securities, Available-for-sale, Fair Value	1,203,495	1,328,039
Agency Mortgage-Backed Securities [Member]
Investment [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	3,791,709	3,253,433
Available For Sale Securities, Gross Unreailzed Gain at Period End	69,002	9,820
Available For Sale Securities, Gross Unrealized Loss at Period End	(2,136)	(48,268)
Debt Securities, Available-for-sale, Fair Value	3,858,575	3,214,985
Non-Agency Mortgage-Backed Securities [Member]
Investment [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	921,080	1,053,854
Available For Sale Securities, Gross Unreailzed Gain at Period End	16,528	6,641
Available For Sale Securities, Gross Unrealized Loss at Period End	(1,031)	(12,779)
Debt Securities, Available-for-sale, Fair Value	936,577	1,047,716
Asset-Backed Securities [Member]
Investment [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	1,288,360	1,518,976
Available For Sale Securities, Gross Unreailzed Gain at Period End	13,238	3,849
Available For Sale Securities, Gross Unrealized Loss at Period End	(2,763)	(11,211)
Debt Securities, Available-for-sale, Fair Value	1,298,835	1,511,614
Total Mortgage And Asset-Backed Securities [Member]
Investment [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	6,001,149	5,826,263
Available For Sale Securities, Gross Unreailzed Gain at Period End	98,768	20,310
Available For Sale Securities, Gross Unrealized Loss at Period End	(5,930)	(72,258)
Debt Securities, Available-for-sale, Fair Value	6,093,987	5,774,315
Other Debt Securities [Member]
Investment [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	338,199	339,595
Available For Sale Securities, Gross Unreailzed Gain at Period End	6,052	72
Available For Sale Securities, Gross Unrealized Loss at Period End	(8)	(7,410)
Debt Securities, Available-for-sale, Fair Value	$ 344,243	$ 332,257

This item represents the gross unrealized gains for securities which are categorized as neither held-to-maturity nor trading securities. Such gross unrealized gains are the excess of the fair value of the Available-for-sale Securities over their carrying value as of the reporting date. Such gross unrealized gains are included in other comprehensive income in the statement of shareholders' equity. This concept represents the balance in unrealized gains at period-end.

No definition available.

+ Details

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

- Definition

This item represents the gross unrealized losses for securities which are categorized neither as held-to-maturity nor trading securities. Such gross unrealized losses are the excess of the carrying value of the Available-for-sale Securities over their fair value as of the reporting date. Such gross unrealized losses are included in other comprehensive income in the statement of shareholders' equity. This concept represents the balance in unrealized loss at period end.

+ References

No definition available.

+ Details

Name:	cbsh_AvailableForSaleSecuritiesGrossUnrealizedLossatPeriodEnd
Namespace Prefix:	cbsh_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

+ Details

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

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26610-111562

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

+ Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasuryAndGovernmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=cbsh_AgencyMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=cbsh_TotalMortgageAndAssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_OtherDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: