Parent Company Condensed Financial Statements (Condensed Statements Of Cash Flows) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Net income	$ 259,623	$ 221,875	$ 168,375
Other changes, net	(2,472)	2,768	(17,310)
Net cash provided by operating activities	407,361	671,177	295,288
(Increase) decrease in securities purchased under agreements to resell	(500,000)	(450,000)
Proceeds from maturities/pay downs of investment securities	2,562,551	2,308,323	1,332,347
Purchases of available for sale securities	(4,517,463)	(3,217,600)	(4,078,962)
Purchases of land, buildings and equipment	(21,332)	(18,528)	(29,247)
Net cash provided by (used in) investing activities	(2,184,835)	(654,454)	(1,572,267)
Purchases of treasury stock	(101,154)	(40,984)	(528)
Issuance under open market stock sale program, stock purchase and equity compensation plans	15,349	11,310	103,641
Net tax benefit related to equity compensation plans	1,065	1,178	557
Cash dividends paid on common stock	(79,140)	(78,231)	(74,720)
Net cash provided by (used in) financing activities	1,834,350	(19,882)	441,457
Increase (decrease) in cash	56,876	(3,159)	(835,522)
Cash and cash equivalents at beginning of year	460,675	463,834	1,299,356
Cash and cash equivalents at end of year	517,551	460,675	463,834
Income tax payments (receipts), net	106,653	100,610	82,900
Commerce Bancshares, Inc. (Parent) [Member]
Net income	256,343	221,710	169,075
Earnings of consolidated subsidiaries, net of dividends	(74,260)	(110,809)	(128,536)
Other changes, net	(1,144)	(4,787)	(1,093)
Net cash provided by operating activities	180,939	106,114	39,446
(Increase) decrease in securities purchased under agreements to resell	(40,375)	(30,175)	18,900
Decrease in investment in subsidiaries, net	116	101	353
Proceeds from sales of investment securities		185	11,812
Proceeds from maturities/pay downs of investment securities	22,233	26,487	105,944
Purchases of available for sale securities		(110)	(195,935)
(Increase) decrease in advances to subsidiaries, net	1,658	2,499	(9,080)
Purchases of land, buildings and equipment	(685)	1,629	(409)
Net cash provided by (used in) investing activities	(17,053)	616	(68,415)
Purchases of treasury stock	(101,154)	(40,984)	(528)
Issuance under open market stock sale program, stock purchase and equity compensation plans	15,349	11,310	103,641
Net tax benefit related to equity compensation plans	1,065	1,178	557
Cash dividends paid on common stock	(79,140)	(78,231)	(74,720)
Net cash provided by (used in) financing activities	(163,880)	(106,727)	28,950
Increase (decrease) in cash	6	3	(19)
Cash and cash equivalents at beginning of year	55	52	71
Cash and cash equivalents at end of year	61	55	52
Income tax payments (receipts), net	$ (2,700)	$ 2,000	$ (4,900)