Employee Benefit Plans - Components of and Weighted-Average Assumptions Used in Determining Net Periodic Benefit Costs (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Components of net periodic benefit cost:
Service cost—benefits earned during year	$ 2,283	$ 2,248	$ 2,835
Interest cost on projected benefit obligation	4,507	5,266	4,517
Expected return on plan assets	(10,433)	(8,869)	(8,939)
Amortization of prior service (credit) cost	(34)	26	26
Amortization of net loss	3,192	3,240	3,053
Net periodic pension (income) cost	(485)	1,911	1,492
Amortization of prior service credit	(3,000)	(3,042)	(2,948)
Other changes in plan assets and benefit obligations recognized in other comprehensive income:
Net loss (gain) for period	432	2,946	2,068
Prior service credit	(313)
Unrecognized loss on merged plan			1,429
Amortization of prior service credit (cost)	34	(26)	(26)
Amortization of net loss	(3,192)	(3,240)	(3,053)
Total recognized in other comprehensive (income) loss	(3,039)	(320)	418
Total recognized in net periodic pension cost and other comprehensive income	(3,524)	1,591	1,910
Net (gain) loss for the period	(432)	(2,946)	(2,068)
Amortization of prior service credit	$ (34)	$ 26	$ 26
Weighted-average assumptions used to determine net periodic pension cost:
Discount rate	3.38%	4.48%	3.81%
Rate of compensation increase	3.53%	3.62%	3.70%
Expected long-term return on assets	6.30%	6.30%	6.30%
Farmers Capital Bank Corporation Postretirement Medical Benefit Plan [Member]
Components of net periodic benefit cost:
Interest cost on projected benefit obligation	$ 360	$ 460
Amortization of net loss	67
Net periodic pension (income) cost	203	236
Amortization of prior service credit	(224)	(224)
Other changes in plan assets and benefit obligations recognized in other comprehensive income:
Net loss (gain) for period	(302)	(1,304)
Amortization of prior service credit (cost)	(224)	(224)
Amortization of net loss	(67)
Total recognized in other comprehensive (income) loss	459	1,528
Total recognized in net periodic pension cost and other comprehensive income	662	1,764
Net (gain) loss for the period	302	1,304
Amortization of prior service credit	$ 224	$ 224
Weighted-average assumptions used to determine net periodic pension cost:
Discount rate	1.97%	2.96%