Impact of the Tax Cuts and Jobs Act of 2017 (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2018	Sep. 30, 2017
Income Tax Examination [Line Items]
Effective federal tax rate			24.50%
Reduction in deferred tax liability			$ 903,700
Regulated operations recorded as a regulatory liability	$ 738,200		738,200
Regulatory liability amortized	500
One-time tax benefit			165,500
Remeasured federal net operating loss carryforwards	270,700		270,700
Federal alternative minimum tax credit carryforwards	10,100		10,100
Remeasured charitable contribution carryforwards	5,300		5,300
State net operating loss carryforwards	21,200		21,200
State tax credit carryforwards	1,500		1,500
State net operating loss carryforwards, federal effects	5,600		5,600
State tax credit carryforwards, federal effects	400		400
Valuation allowance reduction		$ 4,200
Regulatory liability	1,499,747		$ 1,499,747	$ 555,657
Minimum
Income Tax Examination [Line Items]
Return issuance period			18 years
Maximum
Income Tax Examination [Line Items]
Return issuance period			40 years
Regulatory cost of service reserve
Income Tax Examination [Line Items]
Regulatory liability	$ 30,930		$ 30,930	$ 0