SCHEDULE I - CONDENSED FINANCIAL INFORMATION OF PARENT 4 (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Note payable to banks
Maximum borrowing capacity	$ 150,000,000.0
Short-term borrowing activities (excluding letters of credit)
Balance outstanding at the end of the period	$ 0	$ 59,000,000
Short-term borrowing (excluding letters of credit)
Short-term borrowing activities (excluding letters of credit)
Balance outstanding at the end of the period		$ 59,000,000
Interest Rate at the end of the period (as a percent)	3.19%	2.28%
Average Amount Outstanding	$ 69,559,000	$ 65,242,000
Weighted Average Annual Interest Rate (as a percent)	2.66%	1.69%
Maximum Amount Outstanding	$ 95,500,000	$ 102,500,000
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	325.00%
Syndicated revolving credit facility | Maximum
Short-term borrowing activities (excluding letters of credit)
Total funded debt ratio	65.00%
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	150,000,000
Short-term borrowing activities (excluding letters of credit)
Balance outstanding at the end of the period	$ 0	59,000,000
Parent Company | Syndicated revolving credit facility
Short-term borrowing activities (excluding letters of credit)
Interest coverage ratio	623.00%
Total funded debt ratio	43.00%
Parent Company | Syndicated revolving credit facility | Minimum
Short-term borrowing activities (excluding letters of credit)
Interest coverage ratio	325.00%
Total funded debt ratio	65.00%
Parent Company | Syndicated revolving credit facility | Maximum
Short-term borrowing activities (excluding letters of credit)
Total funded debt ratio	100.00%
Parent Company | Syndicated revolving credit facility | Letters of credit
Note payable to banks
Maximum borrowing capacity	$ 25,000,000
Letter of credit, amount	$ 900,000
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	(79,000,000)	(36,500,000)	$ (33,800,000)
GSWC
Short-term borrowing activities (excluding letters of credit)
Dividends paid	(68,850,000)	(27,680,000)	$ (25,450,000)
Syndicated revolving credit facility | Syndicated revolving credit facility
Note payable to banks
Maximum borrowing capacity	$ 150,000,000
Notes Receivable | Notes Payable, Other Payables [Member] | GSWC
Short-term borrowing activities (excluding letters of credit)
Notes Receivable, Related Parties, Current		$ 0