STOCK-BASED COMPENSATION PLAN - Nonvested Restricted Stock Awards and Fair Value Assumptions (Details)	9 Months Ended
	Sep. 30, 2021 $ / shares shares
Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares outstanding (in shares) | shares	54,419
Fair value per share (in dollars per share) | $ / shares	$ 24.75
2019 - TSR | Officers & Key Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares outstanding (in shares) | shares	32,292
Fair value per share (in dollars per share) | $ / shares	$ 32.88
Expected Volatility	23.20%
Risk-Free Interest Rate	2.40%
2019 - CEGR, Time | Officers & Key Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares outstanding (in shares) | shares	69,265
Fair value per share (in dollars per share) | $ / shares	$ 31.38
2020 - TSR | Officers & Key Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares outstanding (in shares) | shares	38,813
Fair value per share (in dollars per share) | $ / shares	$ 25.51
Expected Volatility	34.80%
Risk-Free Interest Rate	0.21%
2020 - CEGR, Time | Officers & Key Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares outstanding (in shares) | shares	123,686
Fair value per share (in dollars per share) | $ / shares	$ 25.19
2021 - TSR | Officers & Key Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares outstanding (in shares) | shares	42,716
Fair value per share (in dollars per share) | $ / shares	$ 28.11
Expected Volatility	39.90%
Risk-Free Interest Rate	0.27%
2021 - CEGR, Time | Officers & Key Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares outstanding (in shares) | shares	186,746
Fair value per share (in dollars per share) | $ / shares	$ 25.33