SCHEDULE I - CONDENSED FINANCIAL INFORMATION OF PARENT 4 (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2020	Jun. 30, 2020	Feb. 11, 2020	Mar. 31, 2019
Note payable to banks
Notes Payable to Bank	$ 205,000,000.0
Maximum borrowing capacity	225,000,000.0
Short-term borrowing activities (excluding letters of credit)
Notes payable to banks	$ 5,000,000	$ 0
Short-term borrowing (excluding letters of credit)
Short-term borrowing activities (excluding letters of credit)
Notes payable to banks		$ 95,500,000
Interest Rate at the end of the period (as a percent)	2.44%	3.19%
Average Amount Outstanding	$ 167,392,000	$ 69,559,000
Weighted Average Annual Interest Rate (as a percent)	2.88%	2.66%
Maximum Amount Outstanding	$ 205,500,000	$ 95,500,000
Long-term Debt [Member]
Note payable to banks
Notes Payable to Bank	$ 205,000,000.0
Syndicated revolving credit facility
Note payable to banks
Variable rate basis	Euro rate
Syndicated revolving credit facility | Minimum
Short-term borrowing activities (excluding letters of credit)
Interest coverage ratio	3.25
Syndicated revolving credit facility | Maximum
Short-term borrowing activities (excluding letters of credit)
Total funded debt ratio	0.65
Syndicated revolving credit facility | Letter of Credit Irrevocable Franchise Agreement with City of Rancho Cordova [Member]
Note payable to banks
Letter of credit, amount	$ 15,000,000
Parent Company
Note payable to banks
Maximum borrowing capacity	225,000,000.0
Short-term borrowing activities (excluding letters of credit)
Notes payable to banks	$ 5,000,000	0
Parent Company | Syndicated revolving credit facility
Short-term borrowing activities (excluding letters of credit)
Interest coverage ratio	6.89
Total funded debt ratio	0.45
Parent Company | Syndicated revolving credit facility | Minimum
Short-term borrowing activities (excluding letters of credit)
Interest coverage ratio	3.25
Total funded debt ratio	0.65
Parent Company | Syndicated revolving credit facility | Maximum
Short-term borrowing activities (excluding letters of credit)
Total funded debt ratio	1.00
Parent Company | Syndicated revolving credit facility | Letters of credit
Note payable to banks
Maximum borrowing capacity	$ 25,000,000.0
Letter of credit, amount	$ 940,000
Letter of credit fee (as a percent)	0.65%
Parent Company | Syndicated revolving credit facility | Letter of Credit - GSWC business automobile insurance policy
Note payable to banks
Letter of credit, amount	$ 340,000
Parent Company | Syndicated revolving credit facility | Letter of Credit - Purchase of power
Note payable to banks
Letter of credit, amount	585,000,000
Subsidiaries [Member]
Short-term borrowing activities (excluding letters of credit)
Dividends paid	(42,700,000)	(79,000,000.0)	$ (36,500,000)
GSWC
Short-term borrowing activities (excluding letters of credit)
Dividends paid	$ (20,200,000)	$ (68,850,000)	$ (27,680,000)
Syndicated revolving credit facility | Parent Company
Note payable to banks
Maximum borrowing capacity							$ 200,000,000.0
Syndicated revolving credit facility | Parent Company | Minimum
Note payable to banks
Line of Credit Facility, Current Borrowing Capacity							$ 150,000,000.0
Subsequent Event [Member] | Parent Company
Note payable to banks
Maximum borrowing capacity				$ 200,000,000.0	$ 200,000,000.0	$ 260,000,000.0