Income Taxes (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Income Taxes [Line Items]
Tax benefit recorded from exercise of stock options	$ 1,900	$ 9,100	$ 4,600
Tax benefit not realized due to tax loss carryforwards	32,600	32,800
Taxes Refundable to Customers	93,318	89,448
Recoverable Future Taxes	177,261	168,214
Tax Credit Carryforwards	49,000
Federal net operating loss, certain annual limitations	4,500
Federal net operating loss, excess tax deductions related to stock based compensation	85,000
Tax benefit from change in enacted rate	81,976	113,251	$ (26,272)
Deferred Income Taxes [Member]
Income Taxes [Line Items]
Taxes Refundable to Customers	93,300	89,400
Recoverable Future Taxes	177,300	168,200
Regulatory liabilities	52,600	52,600
Regulatory assets	94,200	$ 88,700
Pennsylvania [Member]
Income Taxes [Line Items]
Net operating loss	332,000
California [Member]
Income Taxes [Line Items]
Net operating loss	184,000
New York [Member]
Income Taxes [Line Items]
Net operating loss	80,000
Federal [Member]
Income Taxes [Line Items]
Net operating loss	379,000
State Tax Law Change [Member]
Income Taxes [Line Items]
Tax benefit from change in enacted rate	$ 2,800
New York [Member] | State and Local Jurisdiction [Member]
Income Taxes [Line Items]
Corporate Tax Rate	7.10%
Tax Year 2016 [Member] | New York [Member] | State and Local Jurisdiction [Member]
Income Taxes [Line Items]
Corporate Tax Rate	6.50%
Research and Development Tax Credit Carryforward [Member]
Income Taxes [Line Items]
Tax Credit Carryforwards	$ 5,100