Income Taxes - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2014
Income Taxes
Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Change In Tax Rate, Deferred Tax Liability, Provisional Income Tax (Expense) Benefit, Including Unrecognized Deferred Tax Liability		$ 296,677
Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Change In Tax Rate, Deferred Tax Liability, Provisional Income Tax Expense (Benefit)				$ (7,315)
Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Change In Tax Rate, Provisional Unrecognized Deferred Tax Liability		$ 205,759
Increase (Decrease) In Regulatory Liabilities, Excess Deferred Income Taxes			$ 83,603
PUC Temporary Rates Order, Reduction In Revenue	$ 24,098			24,430	$ 0	$ 0
Interest (income) expense				6	(73)	204
Unrecognized tax benefits				688	1,829	2,055	$ 0
Accrued interest				6
Unrecognized tax benefits if recognized would impact the reported effective tax rate				688
State and Local Jurisdiction
Income Taxes
Decrease in income tax expense due to state tax flow through of accelerated depreciation				$ 4,211	$ 2,537	$ 1,344
Income Tax Benefits
Income Taxes
PUC Temporary Rates Order, Reduction In Revenue			17,135
Future Amortization Of Regulatory Liabilities
Income Taxes
PUC Temporary Rates Order, Reduction In Revenue			$ 6,963