EMPLOYEE BENEFITS - Salary Continuation Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Northern California Salary Continuation
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Postemployment benefits expense	$ 0.2	$ 0.1	$ 0.1
Postemployment benefits liability	0.9	0.9
Cash surrender value of life insurance	6.6	6.4
Southern California Salary Continuation Plan
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Postemployment benefits expense	0.2	0.1	$ 0.1
Postemployment benefits liability	1.1	1.3
Cash surrender value of life insurance	$ 5.0	$ 5.0