Investment Securities (Available For Sale Securities Unrealized Gains And Losses, By Security Type) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Investment [Line Items]
Amortized Cost	$ 9,386,302	$ 8,874,540
Available For Sale Securities, Gross Unreailzed Gain at Period End	172,642	159,318
Available For Sale Securities, Gross Unrealized Loss at Period End	(35,384)	(118,178)
Fair Value	9,523,560us-gaap_AvailableForSaleSecurities	8,915,680us-gaap_AvailableForSaleSecurities
U.S. Government And Federal Agency Obligations [Member]
Investment [Line Items]
Amortized Cost	497,336	498,226
Available For Sale Securities, Gross Unreailzed Gain at Period End	9,095	20,614
Available For Sale Securities, Gross Unrealized Loss at Period End	(5,024)	(13,144)
Fair Value	501,407	505,696
Government-sponsored enterprise obligations [Member]
Investment [Line Items]
Amortized Cost	968,574	766,802
Available For Sale Securities, Gross Unreailzed Gain at Period End	2,593	2,245
Available For Sale Securities, Gross Unrealized Loss at Period End	(8,040)	(27,281)
Fair Value	963,127	741,766
State and municipal obligations [Member]
Investment [Line Items]
Amortized Cost	1,789,215	1,624,195
Available For Sale Securities, Gross Unreailzed Gain at Period End	32,340	28,321
Available For Sale Securities, Gross Unrealized Loss at Period End	(8,354)	(33,345)
Fair Value	1,813,201	1,619,171
Agency mortgage-backed securities [Member]
Investment [Line Items]
Amortized Cost	2,523,377	2,743,803
Available For Sale Securities, Gross Unreailzed Gain at Period End	75,923	54,659
Available For Sale Securities, Gross Unrealized Loss at Period End	(5,592)	(26,124)
Fair Value	2,593,708	2,772,338
Non-agency mortgage-backed securities [Member]
Investment [Line Items]
Amortized Cost	372,911	236,595
Available For Sale Securities, Gross Unreailzed Gain at Period End	11,061	12,008
Available For Sale Securities, Gross Unrealized Loss at Period End	(1,228)	(1,620)
Fair Value	382,744	246,983
Other asset-backed securities [Member]
Investment [Line Items]
Amortized Cost	3,090,174	2,847,368
Available For Sale Securities, Gross Unreailzed Gain at Period End	6,922	6,872
Available For Sale Securities, Gross Unrealized Loss at Period End	(5,103)	(10,169)
Fair Value	3,091,993	2,844,071
Total mortgage and asset-backed securities [Member]
Investment [Line Items]
Amortized Cost	5,986,462	5,827,766
Available For Sale Securities, Gross Unreailzed Gain at Period End	93,906	73,539
Available For Sale Securities, Gross Unrealized Loss at Period End	(11,923)	(37,913)
Fair Value	6,068,445	5,863,392
Other debt securities [Member]
Investment [Line Items]
Amortized Cost	140,784	147,581
Available For Sale Securities, Gross Unreailzed Gain at Period End	420	671
Available For Sale Securities, Gross Unrealized Loss at Period End	(2,043)	(6,495)
Fair Value	139,161	141,757
Equity securities [Member]
Investment [Line Items]
Amortized Cost	3,931	9,970
Available For Sale Securities, Gross Unreailzed Gain at Period End	34,288	33,928
Available For Sale Securities, Gross Unrealized Loss at Period End	0	0
Fair Value	$ 38,219	$ 43,898

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

- 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=28360136&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=27724398&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=6871852&loc=d3e26610-111562

+ Details

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

- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.2)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=cbsh_AgencyMortgageBackedSecuritiesMember
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:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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