Income Taxes (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Income Taxes [Line Items]
Tax benefit recorded from exercise of stock options	$ 9,100	$ 4,600	$ 700
Tax benefit not realized due to tax loss carryforwards	32,800	34,200
Taxes Refundable to Customers	89,448	91,736
Recoverable Future Taxes	168,214	163,485
Tax Credit Carryforwards	54,000
Federal net operating loss, certain annual limitations	7,300
Federal net operating loss, excess tax deductions related to stock based compensation	87,000
Tax benefit from change in enacted rate	113,251	(26,272)	$ (37,569)
Deferred Income Taxes [Member]
Income Taxes [Line Items]
Taxes Refundable to Customers	89,400	91,700
Recoverable Future Taxes	168,200	163,500
Regulatory liabilities	52,600	52,600
Regulatory assets	88,700	$ 85,300
Pennsylvania [Member]
Income Taxes [Line Items]
Net operating loss	333,000
California [Member]
Income Taxes [Line Items]
Net operating loss	207,000
New York [Member]
Income Taxes [Line Items]
Net operating loss	96,000
Federal [Member]
Income Taxes [Line Items]
Net operating loss	343,000
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%