Summary Of Significant Accounting Policies (Narrative) (Details) (USD $)	3 Months Ended		6 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Mar. 31, 2012	Jun. 30, 2012	Sep. 30, 2012	Sep. 30, 2011
Summary Of Significant Accounting Policies [Line Items]
Revisions to increase operating cash flows and decrease investing cash flows		$ 16,400,000	$ 17,700,000	$ 32,800,000
Revisions to decrease operating cash flows and increase investing cash flows					1,800,000	6,600,000
Other Accruals and Current Liabilities	103,582,000				79,099,000
Other current assets	49,108,000				56,121,000
Hedging collateral deposits	0				364,000
Gas stored underground - current	(44,485,000)				(49,795,000)
Capitalized costs of unproved properties excluded from amortization	155,600,000				146,100,000
Full cost ceiling test discount factor	10.00%
Amount full cost ceiling exceeds book value of oil and gas properties	7,300,000
Antidilutive securities	422,681	191,285
Reserve For Gas Replacement [Member]
Summary Of Significant Accounting Policies [Line Items]
Gas stored underground - current	6,718,000				0
Stock Options [Member]
Summary Of Significant Accounting Policies [Line Items]
Stock options granted	0
Non-performance Based Stock Appreciation Right [Member]
Summary Of Significant Accounting Policies [Line Items]
Share based compensation other than options grants in period	412,970
Share based compensation arrangement by share based payment award other than options grants in period weighted average grant date excise price	$ 53.05
Granted in fiscal year, weighted average grant date fair value	$ 10.66
Non-performance Based Restricted Stock Units [Member]
Summary Of Significant Accounting Policies [Line Items]
Share based compensation other than options grants in period	26,100
Granted in fiscal year, weighted average grant date fair value	$ 47.20
Performance Based Restriced Stock Units [Member]
Summary Of Significant Accounting Policies [Line Items]
Share based compensation other than options grants in period	210,206
Granted in fiscal year, weighted average grant date fair value	$ 48.49
Exchange-Traded Futures Contracts [Member]
Summary Of Significant Accounting Policies [Line Items]
Hedging collateral deposits	$ 0				$ 400,000