Note 3 - Investment Securities - General Obligation and Revenue Bonds (Details)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
General Obligation Bonds [Member]
Number of issuers	131	116
Amortized cost	$ 107,337,076	$ 117,205,307
Other Investments	107,978,015	116,472,635
Average exposure per issuer (fair value)	824,260	1,004,074
Fair value	$ 107,978,015	$ 116,472,635
General Obligation Bonds [Member] | Illinois [Member]
Number of issuers	20	19
Amortized cost	$ 19,328,700	$ 29,214,559
Other Investments	19,514,024	29,308,438
Average exposure per issuer (fair value)	975,701	1,542,549
Fair value	$ 19,514,024	$ 29,308,438
General Obligation Bonds [Member] | Iowa [Member]
Number of issuers	16	27
Amortized cost	$ 13,881,689	$ 32,258,612
Other Investments	13,969,512	32,231,936
Average exposure per issuer (fair value)	873,095	1,193,775
Fair value	$ 13,969,512	$ 32,231,936
General Obligation Bonds [Member] | Missouri [Member]
Number of issuers	17	14
Amortized cost	$ 9,243,355	$ 8,291,192
Other Investments	9,308,287	8,323,245
Average exposure per issuer (fair value)	547,546	594,518
Fair value	$ 9,308,287	$ 8,323,245
General Obligation Bonds [Member] | North Dakota [Member]
Number of issuers	7	7
Amortized cost	$ 21,626,574	$ 22,169,050
Other Investments	21,724,197	21,499,075
Average exposure per issuer (fair value)	3,103,457	3,071,296
Fair value	$ 21,724,197	$ 21,499,075
General Obligation Bonds [Member] | Ohio [Member]
Number of issuers	9	8
Amortized cost	$ 8,002,705	$ 6,790,398
Other Investments	7,938,028	6,651,897
Average exposure per issuer (fair value)	882,003	831,487
Fair value	$ 7,938,028	$ 6,651,897
General Obligation Bonds [Member] | Texas [Member]
Number of issuers	17
Amortized cost	$ 11,253,775
Other Investments	11,308,848
Average exposure per issuer (fair value)	665,226
Fair value	$ 11,308,848
General Obligation Bonds [Member] | Other States [Member]
Number of issuers	45	41
Amortized cost	$ 24,000,278	$ 18,481,496
Other Investments	24,215,119	18,458,044
Average exposure per issuer (fair value)	538,114	450,196
Fair value	$ 24,215,119	$ 18,458,044
Revenue Bonds [Member]
Number of issuers	145	120
Amortized cost	$ 337,263,493	$ 257,470,290
Other Investments	337,347,285	255,496,690
Average exposure per issuer (fair value)	2,326,533	2,129,139
Fair value	$ 337,347,285	$ 255,496,690
Revenue Bonds [Member] | Illinois [Member]
Number of issuers	2
Amortized cost	$ 17,211,441
Other Investments	17,408,544
Average exposure per issuer (fair value)	8,704,272
Fair value	$ 17,408,544
Revenue Bonds [Member] | Indiana [Member]
Number of issuers	26	22
Amortized cost	$ 51,171,818	$ 47,994,737
Other Investments	50,861,336	47,582,138
Average exposure per issuer (fair value)	1,956,205	2,162,824
Fair value	$ 50,861,336	$ 47,582,138
Revenue Bonds [Member] | Iowa [Member]
Number of issuers	29	31
Amortized cost	$ 68,724,899	$ 70,788,393
Other Investments	69,079,470	71,142,393
Average exposure per issuer (fair value)	2,382,051	2,294,916
Fair value	$ 69,079,470	$ 71,142,393
Revenue Bonds [Member] | Missouri [Member]
Number of issuers	56	47
Amortized cost	$ 106,259,015	$ 90,784,441
Other Investments	106,232,837	89,664,013
Average exposure per issuer (fair value)	1,897,015	1,907,745
Fair value	$ 106,232,837	$ 89,664,013
Revenue Bonds [Member] | Kansas [Member]
Number of issuers	6	6
Amortized cost	$ 12,873,329	$ 13,476,366
Other Investments	12,877,087	13,427,491
Average exposure per issuer (fair value)	2,146,181	2,237,915
Fair value	$ 12,877,087	$ 13,427,491
Revenue Bonds [Member] | North Dakota [Member]
Number of issuers	5	4
Amortized cost	$ 11,451,560	$ 8,089,067
Other Investments	11,351,676	7,796,381
Average exposure per issuer (fair value)	2,270,335	1,949,095
Fair value	$ 11,351,676	$ 7,796,381
Revenue Bonds [Member] | Ohio [Member]
Number of issuers	10	3
Amortized cost	$ 55,766,091	$ 13,650,000
Other Investments	55,820,203	13,405,222
Average exposure per issuer (fair value)	5,582,020	4,468,407
Fair value	$ 55,820,203	$ 13,405,222
Revenue Bonds [Member] | Other States [Member]
Number of issuers	11	7
Amortized cost	$ 13,805,340	$ 12,687,286
Other Investments	13,716,132	12,479,052
Average exposure per issuer (fair value)	1,246,921	1,782,722
Fair value	$ 13,716,132	$ 12,479,052