FAIR VALUE MEASUREMENTS - Nonrecurring - Impaired loans measured at FV (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2025	Dec. 31, 2024
FAIR VALUE MEASUREMENTS
Minimum impaired loan balance that requires an appraisal to be obtained and reviewed annually for impaired loan valuation procedure	$ 250,000
Maximum impaired loan balance for which the bank completes a Certificate of Inspection	250,000
Transfer in or out of Level 3	0	$ 0
Fair Value Measurements, Nonrecurring
FAIR VALUE MEASUREMENTS
Total individually evaluated loans	1,603,000	1,603,000
Fair Value, Inputs, Level 1 \| Fair Value Measurements, Nonrecurring
FAIR VALUE MEASUREMENTS
Total individually evaluated loans	0	0
Fair Value, Inputs, Level 2 \| Fair Value Measurements, Nonrecurring
FAIR VALUE MEASUREMENTS
Total individually evaluated loans	0	0
Fair Value, Inputs, Level 3 \| Fair Value Measurements, Nonrecurring
FAIR VALUE MEASUREMENTS
Total individually evaluated loans	1,603,000	1,603,000
Real Estate \| Fair Value Measurements, Nonrecurring
FAIR VALUE MEASUREMENTS
Total individually evaluated loans	1,603,000	1,603,000
Real Estate \| Fair Value, Inputs, Level 1 \| Fair Value Measurements, Nonrecurring
FAIR VALUE MEASUREMENTS
Total individually evaluated loans	0	0
Real Estate \| Fair Value, Inputs, Level 2 \| Fair Value Measurements, Nonrecurring
FAIR VALUE MEASUREMENTS
Total individually evaluated loans	0	0
Real Estate \| Fair Value, Inputs, Level 3 \| Fair Value Measurements, Nonrecurring
FAIR VALUE MEASUREMENTS
Total individually evaluated loans	$ 1,603,000	$ 1,603,000

FIRST KEYSTONE CORP 10-Q