New Accounting Pronouncements - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2012
Change in Accounting Estimate [Line Items]
Fair value measurement, amount	$ 6.9
Fair value measurement, net	$ 4.7

Change in Accounting Estimate Financial Effect, Before Tax

No definition available.

Name:	umbf_ChangeInAccountingEstimateFinancialEffectBeforeTax
Namespace Prefix:	umbf_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration