INCOME TAXES - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Developer deposits for contributions in aid of construction	$ 29,491	$ 25,114
Net operating loss carryforward and tax credits	37,326	11,029
Pension liability	12,031	10,095
Income tax regulatory liability	41,151	47,196
Operating leases liabilities	4,372	4,024
Other	2,812	2,975
Total deferred tax assets	127,183	100,433
Deferred tax liabilities:
Property related basis and depreciation differences	350,923	297,470
WRAM/MCBA and interim rates balancing accounts	39,107	17,771
Operating lease-right to use asset	4,362	4,030
Other	8,823	3,752
Total deferred tax liabilities	403,215	323,023
Net deferred tax liabilities	$ 276,032	$ 222,590