Taxes on Income (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Taxes on income
Unrecognized Tax Benefits	$ 0
Deferred tax assets:
Regulatory-liability-related: ITC and excess deferred taxes	32,761,000	$ 903,000
Regulatory-liability-related: California Corp Franchise Tax	1,806,000	3,365,000
Other nonproperty-related	2,509,000	1,993,000
Contributions and advances	4,679,000	7,464,000
Deferred tax assets	41,755,000	13,725,000
Deferred tax liabilities:
Fixed assets	(130,115,000)	(200,378,000)
Regulatory-asset-related: depreciation and other	(16,851,000)	(24,402,000)
Deferred Tax Liabilities, Regulatory Assets and Liabilities Related to State Franchise Tax	(528,000)	(2,033,000)
Other nonproperty-related	(47,000)	0
Balancing and memorandum accounts	(7,311,000)	(7,010,000)
Deferred charges	(2,594,000)	(4,429,000)
Deferred tax liabilities	(157,446,000)	(238,252,000)
Accumulated deferred income taxes - net	(115,691,000)	(224,527,000)
Current
Federal	20,978,000	2,297,000	$ 21,866,000
State	5,844,000	4,798,000	5,442,000
Total current tax expense	26,822,000	7,095,000	27,308,000
Deferred
Federal	11,543,000	26,715,000	8,948,000
State	609,000	925,000	1,475,000
Total deferred tax expense	12,152,000	27,640,000	10,423,000
Reconciliations of the effective tax rates to the federal statutory rate
Federal taxes on pretax income at statutory rate	37,919,000	33,067,000	34,375,000
Flow-through on removal costs	(1,421,000)	(78,000)	(1,560,000)
Effective Income Tax Rate Reconciliation, Tax Credit, Investment, Amount	(93,000)	(83,000)	(88,000)
Income tax expense/(benefit)	38,974,000	34,735,000	37,731,000
Pretax income from continuing operations	$ 108,341,000	$ 94,478,000	$ 98,215,000
Effective income tax rate (as a percent)	36.00%	36.80%	38.40%
Increase (decrease) in taxes resulting from:	$ 4,382,000	$ 3,029,000	$ 4,843,000
Adjustments recorded to revalue deferred taxes as a result of the tax cut and job act of 2017 effective tax rate impact	(82,000)	0	0
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depreciation, Amount	845,000	994,000	626,000
Income Tax Reconciliation Flow Through on Pension Costs	412,000	(247,000)	267,000
Income Tax Reconciliation Cost of Removal	(1,980,000)	(2,068,000)	(929,000)
Reduction to cumulative net deferred income tax liabilities	90,100,000
Income Tax Examination, Penalties Accrued		0	0
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount	(1,008,000)	121,000	197,000
Income Tax Examination, Penalties Expense		0	0
Proceeds from Income Tax Refunds	2,200,000	2,100,000
GSWC
Deferred tax assets:
Regulatory-liability-related: ITC and excess deferred taxes	32,761,000	903,000
Regulatory-liability-related: California Corp Franchise Tax	1,806,000	3,365,000
Other nonproperty-related	2,230,000	1,901,000
Contributions and advances	5,022,000	7,712,000
Deferred tax assets	41,819,000	13,881,000
Deferred tax liabilities:
Fixed assets	(134,437,000)	(203,133,000)
Regulatory-asset-related: depreciation and other	(16,851,000)	(24,402,000)
Deferred Tax Liabilities, Regulatory Assets and Liabilities Related to State Franchise Tax	(552,000)	(2,208,000)
Other nonproperty-related	(53,000)	(68,000)
Balancing and memorandum accounts	(7,897,000)	(7,271,000)
Deferred charges	(2,809,000)	(4,597,000)
Deferred tax liabilities	(162,599,000)	(241,679,000)
Accumulated deferred income taxes - net	(120,780,000)	(227,798,000)
Current
Federal	15,044,000	(3,115,000)	16,196,000
State	5,045,000	3,625,000	5,557,000
Total current tax expense	20,089,000	510,000	21,753,000
Deferred
Federal	11,770,000	25,864,000	8,536,000
State	2,200,000	2,235,000	2,183,000
Total deferred tax expense	13,970,000	28,099,000	10,719,000
Reconciliations of the effective tax rates to the federal statutory rate
Federal taxes on pretax income at statutory rate	30,736,000	26,452,000	28,022,000
Flow-through on removal costs	(1,148,000)	0	(1,268,000)
Effective Income Tax Rate Reconciliation, Tax Credit, Investment, Amount	(93,000)	(82,000)	(88,000)
Income tax expense/(benefit)	34,059,000	28,609,000	32,472,000
Pretax income from continuing operations	$ 87,816,000	$ 75,578,000	$ 80,063,000
Effective income tax rate (as a percent)	38.80%	37.90%	40.60%
Increase (decrease) in taxes resulting from:	$ 4,924,000	$ 3,118,000	$ 5,151,000
Adjustments recorded to revalue deferred taxes as a result of the tax cut and job act of 2017 effective tax rate impact	1,063,000	0	0
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depreciation, Amount	845,000	994,000	626,000
Income Tax Reconciliation Flow Through on Pension Costs	412,000	(247,000)	267,000
Income Tax Reconciliation Cost of Removal	(1,980,000)	(2,068,000)	(929,000)
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount	$ (700,000)	$ 442,000	$ 691,000