Note 22 - Business Segment Information - Selected Financial Information on the Company's Business Segments (Details) - USD ($)		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Total revenue										$ 165,999,610	$ 137,502,624	$ 114,366,574
Net interest income		$ 31,793,353	$ 28,556,348	$ 28,046,697	$ 27,668,883	$ 29,279,665	$ 23,630,777	$ 21,008,813	$ 20,597,522	116,065,281	94,516,777	76,296,724
Provision for loan/lease losses		2,255,381	2,086,436	2,022,993	2,105,109	2,599,345	1,607,986	1,197,850	2,072,985	8,469,919	7,478,166	6,870,900
Net income (loss)		9,901,590	$ 7,853,935	$ 8,766,017	$ 9,184,965	8,529,330	$ 6,107,501	$ 6,676,467	$ 6,373,489	35,706,507	27,686,787	16,927,881
Goodwill		28,334,092				13,110,913				28,334,092	13,110,913	3,222,688
Core deposit intangible		9,078,953				7,381,213				9,078,953	7,381,213	1,471,409
Total assets		3,982,664,773				3,301,943,748				3,982,664,773	3,301,943,748	2,593,198,275
Net income (loss)												16,927,881
Operating Segments [Member] | Commercial Banking Segment [Member] | Quad City Bank and Trust Company [Member]
Total revenue										58,055,715	59,442,052	52,914,705
Net interest income										46,407,078	45,081,080	40,416,563
Provision for loan/lease losses										3,908,919	4,168,166	4,367,234
Net income (loss)										22,095,055	14,116,751
Goodwill		3,222,688				3,222,688				3,222,688	3,222,688	3,222,688
Core deposit intangible
Total assets		1,541,777,558				1,395,785,241				1,541,777,558	1,395,785,241	1,336,571,694
Net income (loss)												10,333,111
Operating Segments [Member] | Commercial Banking Segment [Member] | Cedar Rapids Bank and Trust [Member]
Total revenue										45,367,035	37,242,901	37,593,652
Net interest income										31,042,302	29,205,047	26,635,659
Provision for loan/lease losses										1,050,000	950,000	1,750,000
Net income (loss)										10,712,174	12,317,545
Goodwill		15,223,179								15,223,179
Core deposit intangible		3,693,592				1,271,897				3,693,592	1,271,897	1,471,409
Total assets		1,307,376,687				913,055,738				1,307,376,687	913,055,738	866,872,406
Net income (loss)												7,695,867
Operating Segments [Member] | Commercial Banking Segment [Member] | Guaranty Bank and Trust Company [Member]
Total revenue	[1]									1,806,078
Net interest income	[1]									1,551,356
Provision for loan/lease losses	[1]
Net income (loss)	[1]									346,835
Goodwill	[1]
Core deposit intangible	[1]
Total assets	[1]
Net income (loss)	[1]
Operating Segments [Member] | Commercial Banking Segment [Member] | Community State Bank [Member]
Total revenue										31,944,152	11,406,291
Net interest income										27,020,674	10,004,729
Provision for loan/lease losses										2,783,000	1,460,000
Net income (loss)										7,047,671	2,132,252
Goodwill		9,888,225				9,888,225				9,888,225	9,888,225
Core deposit intangible		5,385,361				6,109,316				5,385,361	6,109,316
Total assets		670,516,373				600,075,798				670,516,373	600,075,798
Net income (loss)
Operating Segments [Member] | Commercial Banking Segment [Member] | Rockford Bank and Trust [Member]
Total revenue										18,035,971	16,043,894	14,816,300
Net interest income										12,707,651	11,887,201	10,854,637
Provision for loan/lease losses										728,000	900,000	753,666
Net income (loss)										2,660,364	3,235,711
Goodwill
Core deposit intangible
Total assets		461,650,765				391,154,780				461,650,765	391,154,780	367,471,639
Net income (loss)												2,402,522
Operating Segments [Member] | Wealth Management Segment [Member]
Total revenue										11,057,519	9,156,948	9,103,173
Net interest income
Provision for loan/lease losses
Net income (loss)										2,241,494	1,665,453
Goodwill
Core deposit intangible
Total assets
Net income (loss)												1,627,586
Operating Segments [Member] | Other Segments [Member]
Total revenue										232,660	109,563	363,432
Net interest income										(2,663,780)	(1,661,280)	(1,610,135)
Provision for loan/lease losses
Net income (loss)										(9,397,086)	(5,780,925)
Goodwill
Core deposit intangible
Total assets		28,267,478				34,998,902				28,267,478	34,998,902	27,605,704
Net income (loss)												(5,131,205)
Intersegment Eliminations [Member]
Total revenue										(499,520)	4,100,975	(424,688)
Net interest income
Provision for loan/lease losses
Net income (loss)
Goodwill
Core deposit intangible
Total assets		$ (26,924,088)				$ (33,126,711)				$ (26,924,088)	$ (33,126,711)	(5,323,168)
Net income (loss)

[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.