Parent Company Condensed Financial Statements (Condensed Statements Of Income) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest and dividends on investment securities	$ 214,689	$ 207,184	$ 191,801
Salaries and employee benefits	448,321	427,310	400,701
Data processing fees paid to affiliates	92,246	92,722	83,944
Community Service	34,377	3,906	3,180
Other	77,459	74,202	73,565
Total non-interest expense	769,684	717,065	676,487
Income tax expense (benefit)	110,506	124,151	116,590
NET INCOME ATTRIBUTABLE TO COMMERCE BANCSHARES, INC.	319,383	275,391	263,730
Commerce Bancshares, Inc. (Parent) [Member]
Dividends received from consolidated subsidiary banks	160,002	160,002	160,001
Earnings of consolidated subsidiaries, net of dividends	147,678	118,704	106,636
Interest and dividends on investment securities	2,099	2,364	2,272
Management fees charged subsidiaries	30,431	30,965	25,713
Investment securities gains (losses)	41,717	1,880	0
Net interest income on advances and note to subsidiaries	514	21	4
Other	3,346	2,720	1,422
Total income	385,787	316,656	296,048
Salaries and employee benefits	33,714	29,116	22,167
Professional fees	2,036	1,951	1,833
Data processing fees paid to affiliates	3,512	3,226	3,186
Community Service	32,093	1,620	991
Other	10,671	9,849	8,278
Total non-interest expense	82,026	45,762	36,455
Income tax expense (benefit)	(15,622)	(4,497)	(4,137)
NET INCOME ATTRIBUTABLE TO COMMERCE BANCSHARES, INC.	$ 319,383	$ 275,391	$ 263,730