Financial Instruments and Risk Management (Derivative Instruments, Notional Amounts) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
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	$ 63
Fair Value Hedging | Expires during 2013 | Crude Oil
Derivative [Line Items]
Total hedged Volumes	9,700,000.0
Fair Value Hedging | Expires during 2014 | Natural Gas
Derivative [Line Items]
Total hedged Volumes	9,700,000,000.0
Fair Value Hedging | Expires during 2014 | Crude Oil
Derivative [Line Items]
Total hedged Volumes	5,400,000.0
Cash Flow Hedges | 2013 | Natural Gas | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	12.7
Average Contract Price, Per Mcf	4.82
Cash Flow Hedges | 2013 | Natural Gas | San Juan Basin
Derivative [Line Items]
Total hedged Volumes	32.8
Average Contract Price, Per Mcf	4.56
Cash Flow Hedges | 2013 | Natural Gas | Permian Basin
Derivative [Line Items]
Total hedged Volumes	4.6
Average Contract Price, Per Mcf	3.45
Cash Flow Hedges | 2013 | Crude Oil | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	8,858.0
Average Contract Price, Per Bbl	90.95
Cash Flow Hedges | 2013 | Crude Oil | WTS/WTI Basis Swaps
Derivative [Line Items]
Total hedged Volumes	3,592.0
Average Contract Price, Per Bbl	(3.03)
Cash Flow Hedges | 2013 | Crude Oil | WTI/WTI Basis Swaps
Derivative [Line Items]
Total hedged Volumes	2,760.0
Average Contract Price, Per Bbl	(1.01)
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	10.6
Average Contract Price, Per Mcf	4.55
Cash Flow Hedges | 2014 | Natural Gas | San Juan Basin
Derivative [Line Items]
Total hedged Volumes	25.7
Average Contract Price, Per Mcf	4.72
Cash Flow Hedges | 2014 | Natural Gas | Permian Basin
Derivative [Line Items]
Total hedged Volumes	9.7
Average Contract Price, Per Mcf	3.81
Cash Flow Hedges | 2014 | Crude Oil | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	9,796.0
Average Contract Price, Per Bbl	92.64
Alabama Gas Corporation | Cash Flow Hedges | 2013 | Natural Gas | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	1.5