Derivatives And Fair Value Measurements (Narrative) (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2013 MMcf	Mar. 31, 2012	Dec. 31, 2010
Realized Gain on Derivative Instruments, Gross, Pretax	$ 9,596,000	$ 5,583,000
Net Deferred Hedge Gain in AOCI to be Released into Earnings Year One	50,420,000
Net Deferred Hedge Gain in AOCI to be Released into Earnings Year Two	37,084,000
Net Deferred Hedge Gain in AOCI to be Released into Earnings Year Three	33,191,000
Net Deferred Hedge Gain in AOCI to be Released into Earnings Year Four	13,476,000
Net Deferred Hedge Gain in AOCI to be Released into Earnings Year Five	12,531,000
Net Deferred Hedge Gain in AOCI to be Released into Earnings Thereafter	41,443,000
Net Deferred Hedge Gain in AOCI to be Released into Earnings	188,145,000
Natural gas hedges original tenure	10 years
Cash received in settlement of hedge			41,500,000
Interest previously accrued and earned			10,700,000
Unrealized Gain (Loss) on Interest Rate Cash Flow Hedges, Pretax, Accumulated Other Comprehensive Income (Loss)			30,800,000
Senior notes maturity			2015
Interest Rate Cash Flow Hedge Gain (Loss) Reclassified to Earnings, Net	1,300,000	1,200,000
Remaining deferred gains on early settlements from interest rate swaps	15,400,000
Gains and losses from the effective portion of derivative assets and liabilities held in AOCI expected to be reclassified into earnings	60,000,000
Net gains resulting from derivative hedge ineffectiveness		$ 5,200,000
Term of derivative instrument	10 years
Gas 2013, MMcfd	200
Gas 2014, MMcfd	170
Gas 2015, MMcfd	150
Gas 2016-2021, MMcfd	40
Fair Value Inputs, Level 3 [Member] | Maximum [Member]
Unobservable inputs included within the fair value calculation	5.79
Fair Value Inputs, Level 3 [Member] | Minimum [Member]
Unobservable inputs included within the fair value calculation	4.02