Taxes on Income (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Taxes on income
Unrecognized Tax Benefits		$ 0	$ 0	$ 0
Deferred tax assets:
Regulatory-liability-related		33,080,000	33,419,000
Contributions and advances		5,777,000	5,281,000
Deferred Tax Assets, Other		5,792,000	2,988,000
Deferred tax assets		44,649,000	41,688,000
Deferred tax liabilities:
Fixed assets		(144,444,000)	(131,413,000)
Regulatory-asset-related: depreciation and other		(20,641,000)	(18,146,000)
Balancing and memorandum accounts (non-flow-through)		(4,868,000)	(6,325,000)
Deferred tax liabilities		(169,953,000)	(155,884,000)
Accumulated deferred income taxes - net		(125,304,000)	(114,196,000)
Current
Federal		12,507,000	17,252,000	20,978,000
State		5,540,000	6,538,000	5,844,000
Total current tax expense		18,047,000	23,790,000	26,822,000
Deferred
Federal		6,407,000	(4,334,000)	11,543,000
State		216,000	(1,439,000)	609,000
Total deferred tax (benefit) expense		6,623,000	(5,773,000)	12,152,000
Reconciliations of the effective tax rates to the federal statutory rate
Federal taxes on pretax income at statutory rate (21% in 2018; 35% in 2017 and 2016)		22,872,000	17,196,000	37,919,000
Increase (decrease) in taxes resulting from:		4,758,000	3,693,000	4,382,000
Change in tax rate		0	(14,000)	(82,000)
Excess deferred tax amortization		(1,579,000)	(2,101,000)	0
Flow-through on fixed assets		1,244,000	429,000	845,000
Flow-through on removal costs		(1,582,000)	(1,445,000)	(1,980,000)
Domestic production activities deduction		0	(26,000)	(1,421,000)
Investment tax credit		(71,000)	(69,000)	(93,000)
Other- net		(972,000)	354,000	(596,000)
Total income tax expense operations		24,670,000	18,017,000	38,974,000
Income before income tax expense		$ 109,012,000	$ 81,888,000	$ 108,341,000
Effective income tax rate (as a percent)		22.60%	22.00%	36.00%
Proceeds from Income Tax Refunds	$ 2,200,000				$ 2,100,000
GSWC
Deferred tax assets:
Regulatory-liability-related		$ 33,080,000	$ 33,419,000
Contributions and advances		6,158,000	5,666,000
Deferred Tax Assets, Other		6,618,000	3,310,000
Deferred tax assets		45,856,000	42,395,000
Deferred tax liabilities:
Fixed assets		(147,759,000)	(135,617,000)
Regulatory-asset-related: depreciation and other		(20,641,000)	(18,146,000)
Balancing and memorandum accounts (non-flow-through)		(5,262,000)	(6,873,000)
Deferred tax liabilities		(173,662,000)	(160,636,000)
Accumulated deferred income taxes - net		(127,806,000)	(118,241,000)
Current
Federal		9,616,000	14,488,000	$ 15,044,000
State		5,480,000	5,932,000	5,045,000
Total current tax expense		15,096,000	20,420,000	20,089,000
Deferred
Federal		4,924,000	(5,531,000)	11,770,000
State		157,000	(1,286,000)	2,200,000
Total deferred tax (benefit) expense		5,081,000	(6,817,000)	13,970,000
Reconciliations of the effective tax rates to the federal statutory rate
Federal taxes on pretax income at statutory rate (21% in 2018; 35% in 2017 and 2016)		18,236,000	12,939,000	30,736,000
Increase (decrease) in taxes resulting from:		4,656,000	3,335,000	4,924,000
Change in tax rate		0	0
Excess deferred tax amortization		(1,579,000)	(2,101,000)	0
Flow-through on fixed assets		1,244,000	429,000	845,000
Flow-through on removal costs		(1,582,000)	(1,445,000)	(1,980,000)
Domestic production activities deduction		0	(25,000)	(1,148,000)
Investment tax credit		(71,000)	(69,000)	(93,000)
Other- net		(727,000)	540,000	(288,000)
Total income tax expense operations		20,177,000	13,603,000	34,059,000
Income before income tax expense		86,840,000	61,615,000	87,816,000
Pretax income from operations		$ 86,840,000	$ 61,615,000	$ 87,816,000
Effective income tax rate (as a percent)		23.20%	22.10%	38.80%