Parent Company Condensed Financial Statements (Condensed Statements Of Cash Flows) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
OPERATING ACTIVITIES:
Net income	$ 438,214	$ 319,900	$ 276,854
Other changes, net	(5,992)	(13,259)	(7,460)
Net cash provided by (used in) operating activities	552,660	426,527	454,963
INVESTING ACTIVITIES
Proceeds from sales of available for sale securities	708,864	792,380	24,380
Proceeds from maturities/pay downs of available for sale securities	1,510,985	1,899,640	2,032,397
Purchases of investment securities	(2,090,333)	(1,853,817)	(1,988,101)
Purchases of land, buildings and equipment	(33,294)	(30,824)	(24,478)
Net cash provided by (used in) investing activities	(91,024)	220,720	(805,213)
FINANCING ACTIVITIES
Purchases of treasury stock	(75,231)	(17,771)	(39,381)
Cash dividends paid on common stock	(100,238)	(91,619)	(87,070)
Cash dividends paid on preferred stock	(9,000)	(9,000)	(9,000)
Net cash provided by (used in) financing activities	223,252	(924,536)	649,172
Increase (decrease) in cash and cash equivalents	684,888	(277,289)	298,922
Income tax payments (receipts), net	84,172	120,744	119,596
Commerce Bancshares, Inc. (Parent) [Member]
OPERATING ACTIVITIES:
Net income	433,542	319,383	275,391
Earnings of consolidated subsidiaries, net of dividends	(233,785)	(147,678)	(118,704)
Other changes, net	2,505	(11,268)	9,541
Net cash provided by (used in) operating activities	202,262	160,437	166,228
INVESTING ACTIVITIES
(Increase) decrease in securities purchased under agreements to resell	0	155,775	(51,335)
(Increase) decrease in investment in subsidiaries, net	0	11	4
Proceeds from sales of available for sale securities	41,638	11,006	2,949
Proceeds from maturities/pay downs of available for sale securities	1,988	2,295	4,105
Purchases of investment securities	(125)	0	0
Origination of Notes Receivable from Related Parties	0	(50,000)	0
(Increase) decrease in advances to subsidiaries, net	(5,296)	(9,518)	13,507
Purchases of land, buildings and equipment	(133)	(52)	(3)
Net cash provided by (used in) investing activities	38,072	109,517	(30,773)
FINANCING ACTIVITIES
Purchases of treasury stock	(75,231)	(17,771)	(39,381)
Issuance of stock under equity compensation plans	(10)	(8)	(6)
Cash dividends paid on common stock	(100,238)	(91,619)	(87,070)
Cash dividends paid on preferred stock	(9,000)	(9,000)	(9,000)
Net cash provided by (used in) financing activities	(184,479)	(118,398)	(135,457)
Increase (decrease) in cash and cash equivalents	55,855	151,556	(2)
Cash and cash equivalents at beginning of year	151,607	51	53
Cash and cash equivalents at end of year	207,462	151,607	51
Income tax payments (receipts), net	$ (1,965)	$ (8,991)	$ (8,958)