Parent Company Condensed Financial Statements (Condensed Statements Of Cash Flows) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Net income	$ 271,439	$ 259,623	$ 221,875
Other changes, net	(10,794)	(2,472)	2,768
Net cash provided by operating activities	383,112	407,361	671,177
(Increase) decrease in securities purchased under agreements to resell	(575,000)	(500,000)	(450,000)
Proceeds from sales of available for sale securities	16,875	19,833	78,640
Proceeds from maturities/pay downs of available for sale securities	3,080,664	2,562,551	2,308,323
Purchases of investment securities	(3,182,857)	(4,517,463)	(3,217,600)
Purchases of land, buildings and equipment	(34,969)	(21,332)	(18,528)
Net cash provided by (used in) investing activities	(1,160,837)	(2,184,835)	(654,454)
Purchases of treasury stock	(104,909)	(101,154)	(40,984)
Issuance under stock purchase and equity compensation plans	15,588	15,349	11,310
Net tax benefit related to equity compensation plans	2,094	1,065	1,178
Cash dividends paid on common stock	(211,608)	(79,140)	(78,231)
Net cash used in financing activities	1,039,999	1,834,350	(19,882)
Increase (decrease) in cash	262,274	56,876	(3,159)
Cash and cash equivalents at beginning of year	517,551	460,675	463,834
Cash and cash equivalents at end of year	779,825	517,551	460,675
Income tax payments (receipts), net	119,166	106,653	100,610
Commerce Bancshares, Inc. (Parent) [Member]
Net income	269,329	256,343	221,710
Earnings of consolidated subsidiaries, net of dividends	(34,467)	(74,260)	(110,809)
Other changes, net	(7,078)	(1,144)	(4,787)
Net cash provided by operating activities	227,784	180,939	106,114
(Increase) decrease in securities purchased under agreements to resell	50,400	(40,375)	(30,175)
Decrease in investment in subsidiaries, net	1,195	116	101
Proceeds from sales of available for sale securities	346	0	185
Proceeds from maturities/pay downs of available for sale securities	17,063	22,233	26,487
Purchases of investment securities	(2,000)	0	(110)
Decrease in advances to subsidiaries, net	4,136	1,658	2,499
Purchases of land, buildings and equipment	(92)	(685)	1,629
Net cash provided by (used in) investing activities	71,048	(17,053)	616
Purchases of treasury stock	(104,909)	(101,154)	(40,984)
Issuance under stock purchase and equity compensation plans	15,588	15,349	11,310
Net tax benefit related to equity compensation plans	2,094	1,065	1,178
Cash dividends paid on common stock	(211,608)	(79,140)	(78,231)
Net cash used in financing activities	(298,835)	(163,880)	(106,727)
Increase (decrease) in cash	(3)	6	3
Cash and cash equivalents at beginning of year	61	55	52
Cash and cash equivalents at end of year	58	61	55
Income tax payments (receipts), net	$ 523	$ (2,700)	$ 2,000