Income Taxes Income Taxes (Summary of Operating Loss and Tax Credit Carryforwards) (Details) $ in Thousands	Sep. 30, 2017 USD ($)
Federal [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss	$ 184,289
Alternative Minimum Tax Credit [Member]
Operating Loss Carryforwards [Line Items]
Tax Credit Carryforwards	81,683
Research and Development Tax Credit Carryforward [Member]
Operating Loss Carryforwards [Line Items]
Tax Credit Carryforwards	5,694
Enhanced Oil Recovery Credit [Member] | Federal [Member]
Operating Loss Carryforwards [Line Items]
Tax Credit Carryforwards	10,502
Pennsylvania [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss	324,572
California [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss	169,723
California [Member] | Alternative Minimum Tax Credit [Member]
Operating Loss Carryforwards [Line Items]
Tax Credit Carryforwards	5,873
California [Member] | Enhanced Oil Recovery Credit [Member]
Operating Loss Carryforwards [Line Items]
Tax Credit Carryforwards	$ 5,061