Stock-Based Compensation and Directors Stock Purchase Plan (Summary Of The Status Of Nonvested Share Awards) (Details) (USD $)	12 Months Ended
Dec. 31, 2014
Nonvested Share Awards [Roll Forward]
Nonvested, share awards, beginning balance	1,200,750cbsh_NonvestedShareAwardsBalances
Granted, Shares	195,177
Vested, Shares	(107,299)cbsh_NonvestedShareAwardsVestedInPeriod
Forfeited, Shares	(28,689)
Nonvested, share awards, ending balance	1,259,939cbsh_NonvestedShareAwardsBalances
Nonvested Weighted Average Grant Date Fair Value [Roll Forward]
Nonvested, Weighted Average Grant Date Fair Value, Beginning Balance	$ 32.64
Granted, Weighted Average Grant Date Fair Value	$ 42.48
Vested, Weighted Average Grant Date Fair Value	$ 28.51
Forfeited, Weighted Average Grant Date Fair Value	$ 36.85
Nonvested , Weighted Average Grant Date Fair Value, Ending Balance	$ 34.41

Nonvested Share Awards, Beginning and Ending Balances

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

Number of nonvested shares that vested during the reporting period

+ References

No definition available.

+ Details

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Data Type:	xbrli:sharesItemType
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- 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

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Data Type:	num:perShareItemType
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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

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Period Type:	instant

- Definition

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

+ References

No definition available.

+ Details

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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

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Namespace Prefix:	cbsh_
Data Type:	num:perShareItemType
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