Note 2 - Investment Securities - General Obligation and Revenue Bonds (Details)	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
General Obligation Bonds [Member]
Number of issuers	119	116
Amortized cost	$ 100,310,981	$ 117,205,307
Fair value	100,424,010	116,472,635
Average exposure per issuer (fair value)	$ 843,899	$ 1,004,074
General Obligation Bonds [Member] | North Dakota [Member]
Number of issuers	7	7
Amortized cost	$ 21,625,068	$ 22,169,050
Fair value	21,593,895	21,499,075
Average exposure per issuer (fair value)	$ 3,084,842	$ 3,071,296
General Obligation Bonds [Member] | Iowa [Member]
Number of issuers	21	27
Amortized cost	$ 17,990,978	$ 32,258,612
Fair value	17,993,895	32,231,936
Average exposure per issuer (fair value)	$ 856,852	$ 1,193,775
General Obligation Bonds [Member] | Missouri [Member]
Number of issuers	16	14
Amortized cost	$ 8,956,396	$ 8,291,192
Fair value	8,983,511	8,323,245
Average exposure per issuer (fair value)	$ 561,469	$ 594,518
General Obligation Bonds [Member] | Illinois [Member]
Number of issuers	21	19
Amortized cost	$ 18,319,644	$ 29,214,559
Fair value	18,501,854	29,308,438
Average exposure per issuer (fair value)	$ 881,041	$ 1,542,549
General Obligation Bonds [Member] | Ohio [Member]
Number of issuers	9	8
Amortized cost	$ 8,289,069	$ 6,790,398
Fair value	8,180,432	6,651,897
Average exposure per issuer (fair value)	$ 908,937	$ 831,487
General Obligation Bonds [Member] | Texas [Member]
Number of issuers	10
Amortized cost	$ 6,609,101
Fair value	6,543,660
Average exposure per issuer (fair value)	$ 654,366
General Obligation Bonds [Member] | Other States [Member]
Number of issuers	35	41
Amortized cost	$ 18,520,725	$ 18,481,496
Fair value	18,626,763	18,458,044
Average exposure per issuer (fair value)	$ 532,193	$ 450,196
Revenue Bonds [Member]
Number of issuers	132	120
Amortized cost	$ 279,241,658	$ 257,470,290
Fair value	278,725,628	255,496,690
Average exposure per issuer (fair value)	$ 2,111,558	$ 2,129,139
Revenue Bonds [Member] | North Dakota [Member]
Number of issuers	5	4
Amortized cost	$ 11,496,420	$ 8,089,067
Fair value	11,471,544	7,796,381
Average exposure per issuer (fair value)	$ 2,294,309	$ 1,949,095
Revenue Bonds [Member] | Iowa [Member]
Number of issuers	28	31
Amortized cost	$ 62,468,124	$ 70,788,393
Fair value	62,615,875	71,142,393
Average exposure per issuer (fair value)	$ 2,236,281	$ 2,294,916
Revenue Bonds [Member] | Missouri [Member]
Number of issuers	55	47
Amortized cost	$ 107,274,712	$ 90,784,441
Fair value	107,108,520	89,664,013
Average exposure per issuer (fair value)	$ 1,947,428	$ 1,907,745
Revenue Bonds [Member] | Indiana [Member]
Number of issuers	23	22
Amortized cost	$ 50,080,705	$ 47,994,737
Fair value	49,560,095	47,582,138
Average exposure per issuer (fair value)	$ 2,154,787	$ 2,162,824
Revenue Bonds [Member] | Ohio [Member]
Number of issuers	5	3
Amortized cost	$ 19,674,376	$ 13,650,000
Fair value	19,523,478	13,405,222
Average exposure per issuer (fair value)	$ 3,904,696	$ 4,468,407
Revenue Bonds [Member] | Kansas [Member]
Number of issuers	6	6
Amortized cost	$ 12,877,308	$ 13,476,366
Fair value	12,879,871	13,427,491
Average exposure per issuer (fair value)	$ 2,146,645	$ 2,237,915
Revenue Bonds [Member] | Other States [Member]
Number of issuers	10	7
Amortized cost	$ 15,370,013	$ 12,687,286
Fair value	15,566,245	12,479,052
Average exposure per issuer (fair value)	$ 1,556,625	$ 1,782,722