Note 6 - Business Segment Information - Selected Financial Information on the Company's Business Segments (Details) - USD ($)		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Total revenue		$ 48,087,745	$ 38,628,853
Net interest income		32,402,918	27,668,883
Provision		2,539,839	2,105,109
Net income (loss)		10,549,961	9,184,965
Goodwill		28,334,092	13,110,913	$ 28,334,092
Core deposit intangible		8,774,402	7,150,346	9,078,953
Assets, Total		4,026,314,414	3,381,012,618	$ 3,982,664,773
Operating Segments [Member] | Commercial Banking Segment [Member] | Quad City Bank and Trust Company [Member]
Total revenue		15,807,570	13,535,941
Net interest income		12,120,302	11,301,482
Provision		1,120,409	931,109
Net income (loss)		4,457,868	3,655,006
Goodwill		3,222,688	3,222,688
Core deposit intangible
Assets, Total		1,526,829,646	1,442,952,197
Operating Segments [Member] | Commercial Banking Segment [Member] | Cedar Rapids Bank and Trust [Member]
Total revenue		15,997,332	10,386,545
Net interest income		10,835,848	6,974,047
Provision		601,828	250,000
Net income (loss)		4,616,528	2,892,560
Goodwill		15,223,179
Core deposit intangible		3,566,728	1,222,019
Assets, Total		1,331,208,620	929,111,309
Operating Segments [Member] | Commercial Banking Segment [Member] | Community State Bank [Member]
Total revenue	[1]	8,163,323	8,131,706
Net interest income	[1]	6,743,947	7,026,508
Provision		575,602	774,000
Net income (loss)	[1]	1,868,588	1,895,134
Goodwill	[1]	9,888,225	9,888,225
Core deposit intangible	[1]	5,207,674	5,928,327
Assets, Total	[1]	696,978,705	608,431,003
Operating Segments [Member] | Commercial Banking Segment [Member] | Rockford Bank and Trust [Member]
Total revenue		4,997,945	3,947,799
Net interest income		3,465,154	2,968,074
Provision		242,000	150,000
Net income (loss)		741,295	844,569
Goodwill
Core deposit intangible
Assets, Total		468,112,032	398,454,864
Operating Segments [Member] | Wealth Management Segment [Member]
Total revenue		3,189,425	2,701,806
Net interest income
Provision
Net income (loss)		770,865	561,062
Goodwill
Core deposit intangible
Assets, Total
Operating Segments [Member] | Other Segments [Member]
Total revenue		12,532,043	9,876,143
Net interest income		(762,333)	(601,228)
Provision
Net income (loss)		10,514,510	9,184,968
Goodwill
Core deposit intangible
Assets, Total		447,591,932	377,316,912
Intersegment Eliminations [Member]
Total revenue		(12,599,893)	(9,951,087)
Net interest income
Provision
Net income (loss)		(12,419,693)	(9,848,334)
Goodwill
Core deposit intangible
Assets, Total		$ (444,406,521)	$ (375,253,667)

[1] Represents financial results for Guaranty Bank for the period from October 1, 2017 through December 2, 2017, when Guaranty Bank was merged into CRBT.