LOANS, ALLOWANCE FOR LOAN LOSSES AND IMPAIRED LOANS - Impaired Loans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Bank impaired loans
With no related allowance recorded, Recorded investment	$ 27,853	$ 46,357	$ 27,853	$ 46,357	$ 39,245
With an allowance recorded, Recorded investment	9,338	5,332	9,338	5,332	4,152
Recorded investment, Total	37,191	51,689	37,191	51,689	43,397
With no related allowance recorded, unpaid principal balance	27,853	46,357	27,853	46,357	39,245
With an allowance recorded, unpaid principal balance	9,338	5,332	9,338	5,332	4,152
Unpaid principal balance, Total	37,191	51,689	37,191	51,689	43,397
With an allowance recorded, Related allowance	1,363	1,181	1,363	1,181	1,115
With no related allowance recorded, Average recorded investment	30,884	46,445	33,551	52,712	49,157
With an allowance recorded, Average recorded investment	7,146	5,245	6,746	4,732	4,143
Average recorded investment, Total	38,030	51,690	40,297	57,444	53,300
With no related allowance recorded, Interest income recognized	283	602	1,031	1,941	2,658
With an allowance recorded, Interest income recognized	43	55	161	121	177
Interest income recognized, Total	326	657	1,192	2,062	2,835
Commercial/Agricultural Real Estate
Bank impaired loans
With no related allowance recorded, Recorded investment	16,820	30,419	16,820	30,419	24,013
With an allowance recorded, Recorded investment	5,924	3,177	5,924	3,177	2,290
Recorded investment, Total	22,744	33,596	22,744	33,596	26,303
With no related allowance recorded, unpaid principal balance	16,820	30,419	16,820	30,419	24,013
With an allowance recorded, unpaid principal balance	5,924	3,177	5,924	3,177	2,290
Unpaid principal balance, Total	22,744	33,596	22,744	33,596	26,303
With an allowance recorded, Related allowance	1,033	772	1,033	772	698
With no related allowance recorded, Average recorded investment	19,618	30,344	20,417	35,467	32,264
With an allowance recorded, Average recorded investment	3,558	3,597	4,107	2,660	2,217
Average recorded investment, Total	23,176	33,941	24,524	38,127	34,481
With no related allowance recorded, Interest income recognized	172	454	663	1,374	1,894
With an allowance recorded, Interest income recognized	0	48	62	73	100
Interest income recognized, Total	172	502	725	1,447	1,994
C&I/Agricultural operating
Bank impaired loans
With no related allowance recorded, Recorded investment	3,451	6,860	3,451	6,860	6,334
With an allowance recorded, Recorded investment	3,000	1,034	3,000	1,034	781
Recorded investment, Total	6,451	7,894	6,451	7,894	7,115
With no related allowance recorded, unpaid principal balance	3,451	6,860	3,451	6,860	6,334
With an allowance recorded, unpaid principal balance	3,000	1,034	3,000	1,034	781
Unpaid principal balance, Total	6,451	7,894	6,451	7,894	7,115
With an allowance recorded, Related allowance	271	159	271	159	190
With no related allowance recorded, Average recorded investment	3,484	7,070	4,893	8,169	7,906
With an allowance recorded, Average recorded investment	3,041	669	1,891	762	636
Average recorded investment, Total	6,525	7,739	6,784	8,931	8,542
With no related allowance recorded, Interest income recognized	29	35	119	188	284
With an allowance recorded, Interest income recognized	41	0	82	12	22
Interest income recognized, Total	70	35	201	200	306
Residential Mortgage
Bank impaired loans
With no related allowance recorded, Recorded investment	7,291	8,715	7,291	8,715	8,542
With an allowance recorded, Recorded investment	414	1,118	414	1,118	1,079
Recorded investment, Total	7,705	9,833	7,705	9,833	9,621
With no related allowance recorded, unpaid principal balance	7,291	8,715	7,291	8,715	8,542
With an allowance recorded, unpaid principal balance	414	1,118	414	1,118	1,079
Unpaid principal balance, Total	7,705	9,833	7,705	9,833	9,621
With an allowance recorded, Related allowance	59	249	59	249	226
With no related allowance recorded, Average recorded investment	7,498	8,668	7,917	8,705	8,619
With an allowance recorded, Average recorded investment	547	970	747	1,275	1,255
Average recorded investment, Total	8,045	9,638	8,664	9,980	9,874
With no related allowance recorded, Interest income recognized	79	107	240	356	450
With an allowance recorded, Interest income recognized	2	7	17	36	54
Interest income recognized, Total	81	114	257	392	504
Consumer Installment
Bank impaired loans
With no related allowance recorded, Recorded investment	291	363	291	363	356
With an allowance recorded, Recorded investment	0	3	0	3	2
Recorded investment, Total	291	366	291	366	358
With no related allowance recorded, unpaid principal balance	291	363	291	363	356
With an allowance recorded, unpaid principal balance	0	3	0	3	2
Unpaid principal balance, Total	291	366	291	366	358
With an allowance recorded, Related allowance	0	1	0	1	1
With no related allowance recorded, Average recorded investment	284	363	324	371	368
With an allowance recorded, Average recorded investment	0	9	1	35	35
Average recorded investment, Total	284	372	325	406	403
With no related allowance recorded, Interest income recognized	3	6	9	23	30
With an allowance recorded, Interest income recognized	0	0	0	0	1
Interest income recognized, Total	$ 3	$ 6	$ 9	$ 23	$ 31