Fair Value Measurements (Narrative) (Details) - USD ($) $ in Thousands		9 Months Ended
		Jun. 30, 2015	Sep. 30, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Hedging collateral deposits	[1]	$ 11,101	$ 2,734
Assumed 12 month Midway Sunset oil comparison to NYMEX		92.60%
Assumed 12 month minimum Midway Sunset oil comparison to NYMEX		87.80%
Assumed 12 month maximum Midway Sunset oil comparison to NYMEX		96.70%
Fair value measured on recurring basis, net	[1]	$ (573,619)	(140,790)
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Hedging collateral deposits		0	0
Fair value measured on recurring basis, net		(2,471)	(1,368)
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Hedging collateral deposits		11,101	2,734
Fair value measured on recurring basis, net		(332,641)	(68,994)
Fair Value, Inputs, Level 1 [Member] | NYMEX Futures [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Hedging collateral deposits		$ 11,100	$ 2,700
Higher [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Midway Sunset oil comparison to NYMEX Sensitivity percentage points		10.00%
Higher [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of crude oil price swap sensitivity		$ (800)
Lower [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Midway Sunset oil comparison to NYMEX Sensitivity percentage points		10.00%
Lower [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of crude oil price swap sensitivity		$ 800

[1] Netting Adjustments represent the impact of legally-enforceable master netting arrangements that allow the Company to net gain and loss positions held with the same counterparties. The net asset or net liability for each counterparty is recorded as an asset or liability on the Company’s balance sheet.