Stock-Based Compensation (Summary Of The Status Of Nonvested Share Awards) (Details) (USD $)	3 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Nonvested Share Awards [Roll Forward]
Granted, Shares	187,043
Vested, Shares	(124,747)cbsh_NonvestedShareAwardsVestedInPeriod
Forfeited, Shares	(3,486)
Nonvested, Shares, Ending Balance	1,318,499cbsh_NonvestedShareAwardsBalances	1,259,689cbsh_NonvestedShareAwardsBalances
Nonvested, Weighted Average Grant Date Fair Value, Beginning Balance	$ 34.41
Granted , Weighted Average Grant Date Fair Value	$ 41.39
Vested , Weighted Average Grant Date Fair Value	$ 29.26
Forfeited , Weighted Average Grant Date Fair Value	$ 33.41
Nonvested , Weighted Average Grant Date Fair Value, Ending Balance	$ 35.89

Nonvested Share Awards, Beginning and Ending Balances

No definition available.

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Balance Type:	na
Period Type:	instant

- Definition

Number of nonvested shares that were forfeited during the reporting period.

+ References

No definition available.

+ Details

Name:	cbsh_NonvestedShareAwardsForfeitedInPeriod
Namespace Prefix:	cbsh_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of nonvested shares that were granted during the reporting period.

+ References

No definition available.

+ Details

Name:	cbsh_NonvestedShareAwardsGrantedInPeriod
Namespace Prefix:	cbsh_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of nonvested shares that vested during the reporting period

+ References

No definition available.

+ Details

Name:	cbsh_NonvestedShareAwardsVestedInPeriod
Namespace Prefix:	cbsh_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Nonvested Share Awards Vested In Period Weighted Average Grant Date Fair Value

+ References

No definition available.

+ Details

Name:	cbsh_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsNonvestedShareAwardsVestedInPeriodWeightedAverageGrantDateFairValue
Namespace Prefix:	cbsh_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Nonvested Share Awards Weighted Average Grant Date Unit Fair Value

+ References

No definition available.

+ Details

Name:	cbsh_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsNonvestedShareAwardsWeightedAverageGrantDateUnitFairValue
Namespace Prefix:	cbsh_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Share-Based Compensation, Forfeited, Nonvested Share Award, Weighted Average Grant Date Fair Value

+ References

No definition available.

+ Details

Name:	cbsh_ShareBasedCompensationForfeitedNonvestedShareAwardWeightedAverageGrantDateFairValue
Namespace Prefix:	cbsh_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Share Based Compensation, Granted, Nonvested Share Award Weighted Average Grant Date Fair Value

+ References

No definition available.

+ Details

Name:	cbsh_ShareBasedCompensationGrantedNonvestedShareAwardWeightedAverageGrantDateFairValue
Namespace Prefix:	cbsh_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration