Financial Instruments and Risk Management (Derivative Instruments, Notional Amounts) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011
Gain (Loss) on Discontinuation of Cash Flow Hedge Due to Forecasted Transaction Probable of Not Occurring, Net [Abstract]
Discontinued hedge accounting and reclass of gains (losses) after-tax, due to probability that ceraint forecasted volumes would not occur	$ 0.2
Fair Value Hedging | Expires during 2012 | Natural Gas
Derivative [Line Items]
Total hedged Volumes	15,100,000.0
Fair Value Hedging | Expires during 2012 | Crude Oil
Derivative [Line Items]
Total hedged Volumes	4,200,000.0
Fair Value Hedging | Expires during 2013 | Natural Gas
Derivative [Line Items]
Total hedged Volumes	1,600,000.0
Fair Value Hedging | Expires during 2013 | Crude Oil
Derivative [Line Items]
Total hedged Volumes	5,800,000.0
Fair Value Hedging | Expires during 2014 | Crude Oil
Derivative [Line Items]
Total hedged Volumes	2,400,000.0
Cash Flow Hedges | 2012 | Natural Gas | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	11.0
Average Contract Price, Per Mcf	5.07
Cash Flow Hedges | 2012 | Natural Gas | Basin Specific Swaps
Derivative [Line Items]
Total hedged Volumes	29.5
Average Contract Price, Per Mcf	4.60
Cash Flow Hedges | 2012 | Crude Oil | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	6,762.0
Average Contract Price, Per Bbl	88.29
Cash Flow Hedges | 2012 | Crude Oil | Basis Swap
Derivative [Line Items]
Total hedged Volumes	3,124.0
Average Contract Price, Per Bbl		[1]
Cash Flow Hedges | 2012 | Natural Gas Liquids | Liquids Swaps
Derivative [Line Items]
Total hedged Volumes	58.5
Average Contract Price, Per Gal	0.98
Cash Flow Hedges | 2013 | Natural Gas | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	8.8
Average Contract Price, Per Mcf	5.30
Cash Flow Hedges | 2013 | Natural Gas | Basin Specific Swaps
Derivative [Line Items]
Total hedged Volumes	25.1
Average Contract Price, Per Mcf	4.88
Cash Flow Hedges | 2013 | Crude Oil | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	7,643.0
Average Contract Price, Per Bbl	90.03
Cash Flow Hedges | 2013 | Crude Oil | Basis Swap
Derivative [Line Items]
Total hedged Volumes	2,768.0
Average Contract Price, Per Bbl		[1]
Cash Flow Hedges | 2013 | Natural Gas Liquids | Liquids Swaps
Derivative [Line Items]
Total hedged Volumes	44.5
Average Contract Price, Per Gal	1.02
Cash Flow Hedges | 2014 | Natural Gas | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	3.0
Average Contract Price, Per Mcf	5.72
Cash Flow Hedges | 2014 | Natural Gas | Basin Specific Swaps
Derivative [Line Items]
Total hedged Volumes	16.8
Average Contract Price, Per Mcf	5.16
Cash Flow Hedges | 2014 | Crude Oil | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	5,612.0
Average Contract Price, Per Bbl	90.56
Alabama Gas Corporation | Cash Flow Hedges | 2012 | Natural Gas | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	17.2
Alabama Gas Corporation | Cash Flow Hedges | 2013 | Natural Gas | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	1.5

[1] Average contract prices are not meaningful due to the varying nature of each contract.