Financial Instruments and Risk Management (Derivative Instruments, Notional Amounts) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013
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 certain forecasted volumes would not occur	$ 4.5
Fair Value Hedging | Expires during 2014 | Natural Gas
Derivative [Line Items]
Total hedged Volumes	100,000,000.0
Fair Value Hedging | Expires during 2014 | Crude Oil
Derivative [Line Items]
Total hedged Volumes	9,800,000.0
Fair Value Hedging | Expires during 2014 | Natural Gas Liquids
Derivative [Line Items]
Total hedged Volumes	0
Fair Value Hedging | Expires during 2015 | Natural Gas
Derivative [Line Items]
Total hedged Volumes	0
Fair Value Hedging | Expires during 2015 | Crude Oil
Derivative [Line Items]
Total hedged Volumes	5,800,000.0
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	31.4
Average Contract Price, Per Mcf	4.60
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
Cash Flow Hedges | 2015 | Natural Gas | San Juan Basin
Derivative [Line Items]
Total hedged Volumes	6.0
Average Contract Price, Per Mcf	4.07
Cash Flow Hedges | 2015 | Crude Oil | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	5,760.0
Average Contract Price, Per Bbl	88.85