Income Taxes Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Advances and contributions	$ 9,929			$ 9,929	$ 17,196
Unamortized investment tax credit	441			441	644
Pensions and postretirement benefits	21,349			21,349	3,927
California franchise tax	1,058			1,058	2,331
Other	3,110			3,110	4,802
Total deferred tax assets	35,887			35,887	28,900
Deferred tax liabilities:
Utility plant	94,414			94,414	177,338
Pension and postretirement benefits	19,184			19,184	44,737
Investment in stock	1,199			1,199	1,280
Deferred gain and other-property related	5,640			5,640	9,769
Debt reacquisition costs	204			204	347
Other	1,041			1,041	632
Total deferred tax liabilities	(121,682)			(121,682)	(234,103)
Net deferred tax liabilities	85,795			85,795	205,203
Deferred tax liabilities, period increase (decrease)				(119,408)	6,428
Income tax credits and adjustments			$ 880	117
Deferred tax liabilities non-cash items					1,625
Provision for income taxes				35,393	33,542	$ 23,272
Provisional income tax expense deferred taxes revaluation	2,357
2017 Federal and State Repairs and Maintenance Deduction [Member]
Deferred tax liabilities:
Income tax credits and adjustments				20,168
2017 Federal and State Repairs and Maintenance Deduction [Member] | State and Local Jurisdiction
Deferred tax liabilities:
Provision for income taxes				(1,159)
2017 Federal and State Repairs and Maintenance Deduction [Member] | Domestic Tax Authority
Deferred tax liabilities:
Other	7,059			7,059
2016 Federal and State Repairs and Maintenance Deduction [Member]
Deferred tax liabilities:
Income tax credits and adjustments					20,611
2016 Federal and State Repairs and Maintenance Deduction [Member] | State and Local Jurisdiction
Deferred tax liabilities:
Provision for income taxes					(1,184)
2016 Federal and State Repairs and Maintenance Deduction [Member] | Domestic Tax Authority
Deferred tax liabilities:
Other					7,214
California Franchise Tax Board
Deferred tax liabilities:
Income tax refund		$ 1,294
Operating loss carryback		$ 404
Deferred Taxes Related to Pensions, Accrued Vacation and Asset Retirement Obligations
Deferred tax liabilities:
Total deferred tax liabilities	27,833			27,833	$ 47,123
Deferred Income Tax Charge
Deferred tax liabilities:
Deferred tax liabilities, period increase (decrease)				$ 118,972
Provisional income tax expense deferred taxes revaluation	$ 83,666