Note 2 - Investment Securities - General Obligation and Revenue Bonds (Details)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
General Obligation Bonds [Member]
Number of issuers	128	131
Amortized cost	$ 102,757,219	$ 107,337,076
Other Investments, Total	101,343,490	107,978,015
Average exposure per issuer (fair value)	791,746	824,260
Fair value	$ 101,343,490	$ 107,978,015
General Obligation Bonds [Member] | North Dakota [Member]
Number of issuers	7	7
Amortized cost	$ 21,627,097	$ 21,626,574
Other Investments, Total	20,756,566	21,724,197
Average exposure per issuer (fair value)	2,965,224	3,103,457
Fair value	$ 20,756,566	$ 21,724,197
General Obligation Bonds [Member] | Missouri [Member]
Number of issuers	17	17
Amortized cost	$ 8,314,872	$ 9,243,355
Other Investments, Total	8,323,476	9,308,287
Average exposure per issuer (fair value)	489,616	547,546
Fair value	$ 8,323,476	$ 9,308,287
General Obligation Bonds [Member] | Illinois [Member]
Number of issuers	20	20
Amortized cost	$ 18,532,776	$ 19,328,700
Other Investments, Total	18,513,766	19,514,024
Average exposure per issuer (fair value)	925,688	975,701
Fair value	$ 18,513,766	$ 19,514,024
General Obligation Bonds [Member] | Iowa [Member]
Number of issuers	16	16
Amortized cost	$ 13,878,991	$ 13,881,689
Other Investments, Total	13,830,003	13,969,512
Average exposure per issuer (fair value)	864,375	873,095
Fair value	$ 13,830,003	$ 13,969,512
General Obligation Bonds [Member] | Ohio [Member]
Number of issuers	8	9
Amortized cost	$ 7,378,887	$ 8,002,705
Other Investments, Total	7,296,859	7,938,028
Average exposure per issuer (fair value)	912,107	882,003
Fair value	$ 7,296,859	$ 7,938,028
General Obligation Bonds [Member] | Texas [Member]
Number of issuers	17	17
Amortized cost	$ 10,763,333	$ 11,253,775
Other Investments, Total	10,554,430	11,308,848
Average exposure per issuer (fair value)	620,849	665,226
Fair value	$ 10,554,430	$ 11,308,848
General Obligation Bonds [Member] | Other States [Member]
Number of issuers	43	45
Amortized cost	$ 22,261,263	$ 24,000,278
Other Investments, Total	22,068,390	24,215,119
Average exposure per issuer (fair value)	513,218	538,114
Fair value	$ 22,068,390	$ 24,215,119
Revenue Bonds [Member]
Number of issuers	148	145
Amortized cost	$ 336,639,783	$ 337,263,493
Other Investments, Total	334,612,172	337,347,285
Average exposure per issuer (fair value)	2,260,893	2,326,533
Fair value	$ 334,612,172	$ 337,347,285
Revenue Bonds [Member] | North Dakota [Member]
Number of issuers	5	5
Amortized cost	$ 11,220,278	$ 11,451,560
Other Investments, Total	10,644,441	11,351,676
Average exposure per issuer (fair value)	2,128,888	2,270,335
Fair value	$ 10,644,441	$ 11,351,676
Revenue Bonds [Member] | Missouri [Member]
Number of issuers	58	56
Amortized cost	$ 108,311,804	$ 106,259,015
Other Investments, Total	108,445,780	106,232,837
Average exposure per issuer (fair value)	1,869,755	1,897,015
Fair value	$ 108,445,780	$ 106,232,837
Revenue Bonds [Member] | Illinois [Member]
Number of issuers	2	2
Amortized cost	$ 17,205,951	$ 17,211,441
Other Investments, Total	17,373,994	17,408,544
Average exposure per issuer (fair value)	8,686,997	8,704,272
Fair value	$ 17,373,994	$ 17,408,544
Revenue Bonds [Member] | Iowa [Member]
Number of issuers	29	29
Amortized cost	$ 68,370,374	$ 68,724,899
Other Investments, Total	68,236,038	69,079,470
Average exposure per issuer (fair value)	2,352,967	2,382,051
Fair value	$ 68,236,038	$ 69,079,470
Revenue Bonds [Member] | Ohio [Member]
Number of issuers	10	10
Amortized cost	$ 55,757,914	$ 55,766,091
Other Investments, Total	55,699,375	55,820,203
Average exposure per issuer (fair value)	5,569,938	5,582,020
Fair value	$ 55,699,375	$ 55,820,203
Revenue Bonds [Member] | Indiana [Member]
Number of issuers	26	26
Amortized cost	$ 49,258,364	$ 51,171,818
Other Investments, Total	48,833,113	50,861,336
Average exposure per issuer (fair value)	1,878,197	1,956,205
Fair value	$ 48,833,113	$ 50,861,336
Revenue Bonds [Member] | Kansas [Member]
Number of issuers	6	6
Amortized cost	$ 12,651,338	$ 12,873,329
Other Investments, Total	12,172,961	12,877,087
Average exposure per issuer (fair value)	2,028,827	2,146,181
Fair value	$ 12,172,961	$ 12,877,087
Revenue Bonds [Member] | Other States [Member]
Number of issuers	12	11
Amortized cost	$ 13,863,760	$ 13,805,340
Other Investments, Total	13,206,470	13,716,132
Average exposure per issuer (fair value)	1,100,539	1,246,921
Fair value	$ 13,206,470	$ 13,716,132