Note 2 - Investment Securities - Amortized Cost and Fair Values of General Obligation Bonds and Revenue Bonds by Issuer's State (Details)	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
General Obligation Bonds [Member]
Fair value	$ 107,854,685	$ 116,472,635
Average exposure per issuer (fair value)	$ 869,796	$ 1,004,074
Number of issuers	124	116
Amortized cost	$ 107,652,780	$ 117,205,307
General Obligation Bonds [Member] | Iowa [Member]
Fair value	31,242,925	32,231,936
Average exposure per issuer (fair value)	$ 1,201,651	$ 1,193,775
Number of issuers	26	27
Amortized cost	$ 31,131,484	$ 32,258,612
General Obligation Bonds [Member] | Missouri [Member]
Fair value	9,123,733	8,323,245
Average exposure per issuer (fair value)	$ 570,233	$ 594,518
Number of issuers	16	14
Amortized cost	$ 9,078,540	$ 8,291,192
General Obligation Bonds [Member] | North Dakota [Member]
Fair value	21,514,855	21,499,075
Average exposure per issuer (fair value)	$ 3,073,551	$ 3,071,296
Number of issuers	7	7
Amortized cost	$ 21,619,812	$ 22,169,050
General Obligation Bonds [Member] | Illinois [Member]
Fair value	15,051,806	29,308,438
Average exposure per issuer (fair value)	$ 792,200	$ 1,542,549
Number of issuers	19	19
Amortized cost	$ 14,872,773	$ 29,214,559
General Obligation Bonds [Member] | Ohio [Member]
Fair value	8,588,019	6,651,897
Average exposure per issuer (fair value)	$ 858,802	$ 831,487
Number of issuers	10	8
Amortized cost	$ 8,716,542	$ 6,790,398
General Obligation Bonds [Member] | Texas [Member]
Fair value	5,336,188
Average exposure per issuer (fair value)	$ 667,024
Number of issuers	8
Amortized cost	$ 5,347,075
General Obligation Bonds [Member] | Other States [Member]
Fair value	16,997,159	18,458,044
Average exposure per issuer (fair value)	$ 447,294	$ 450,196
Number of issuers	38	41
Amortized cost	$ 16,886,554	$ 18,481,496
Revenue Bonds [Member]
Fair value	272,185,202	255,496,690
Average exposure per issuer (fair value)	$ 2,160,200	$ 2,129,139
Number of issuers	126	120
Amortized cost	$ 273,382,192	$ 257,470,290
Revenue Bonds [Member] | Iowa [Member]
Fair value	57,834,419	71,142,393
Average exposure per issuer (fair value)	$ 2,142,016	$ 2,294,916
Number of issuers	27	31
Amortized cost	$ 57,862,675	$ 70,788,393
Revenue Bonds [Member] | Missouri [Member]
Fair value	106,983,845	89,664,013
Average exposure per issuer (fair value)	$ 1,981,182	$ 1,907,745
Number of issuers	54	47
Amortized cost	$ 107,360,504	$ 90,784,441
Revenue Bonds [Member] | North Dakota [Member]
Fair value	11,676,766	7,796,381
Average exposure per issuer (fair value)	$ 2,335,353	$ 1,949,095
Number of issuers	5	4
Amortized cost	$ 11,729,782	$ 8,089,067
Revenue Bonds [Member] | Indiana [Member]
Fair value	49,578,767	47,582,138
Average exposure per issuer (fair value)	$ 2,360,894	$ 2,162,824
Number of issuers	21	22
Amortized cost	$ 50,114,290	$ 47,994,737
Revenue Bonds [Member] | Ohio [Member]
Fair value	19,493,348	13,405,222
Average exposure per issuer (fair value)	$ 3,898,670	$ 4,468,407
Number of issuers	5	3
Amortized cost	$ 19,691,358	$ 13,650,000
Revenue Bonds [Member] | Kansas [Member]
Fair value	13,179,104	13,427,491
Average exposure per issuer (fair value)	$ 2,196,517	$ 2,237,915
Number of issuers	6	6
Amortized cost	$ 13,178,963	$ 13,476,366
Revenue Bonds [Member] | Other States [Member]
Fair value	13,438,953	12,479,052
Average exposure per issuer (fair value)	$ 1,679,869	$ 1,782,722
Number of issuers	8	7
Amortized cost	$ 13,444,620	$ 12,687,286