Derivative Instruments and Hedging Activities Notional Contracts by Derivative Type (Details)	Dec. 31, 2012 MMBTU
Natural gas - long
economic hedges by derivative type [Line Items]
Portion of future gas production being hedged	96,000,000
WTI crude oil - long
economic hedges by derivative type [Line Items]
Notional contract volumes related to swap contracts serving as hedges	12,930,000
Ultra-low sulfur diesel - short
economic hedges by derivative type [Line Items]
Notional contract volumes related to swap contracts serving as hedges	11,490,000
Unleaded gasoline - short
economic hedges by derivative type [Line Items]
Notional contract volumes related to swap contracts serving as hedges	1,440,000
Commodity price swap (WCS spread) - long
economic hedges by derivative type [Line Items]
Notional contract volumes related to swap contracts serving as hedges	6,022,500
Commodity price swap (WTI) - short
economic hedges by derivative type [Line Items]
Notional contract volumes related to swap contracts serving as hedges	150,000
Commodity price swap (gasoline) - short
economic hedges by derivative type [Line Items]
Notional contract volumes related to swap contracts serving as hedges	192,000
NYMEX futures (WTI) - long
economic hedges by derivative type [Line Items]
Notional contract volumes related to swap contracts serving as hedges	234,000
NYMEX futures (WTI) - short
economic hedges by derivative type [Line Items]
Notional contract volumes related to swap contracts serving as hedges	1,091,000
Physical contracts - long
economic hedges by derivative type [Line Items]
Notional contract volumes related to swap contracts serving as hedges	540,000
Physical contracts - short
economic hedges by derivative type [Line Items]
Notional contract volumes related to swap contracts serving as hedges	540,000
2013 | Natural gas - long
economic hedges by derivative type [Line Items]
Portion of future gas production being hedged	19,200,000
2013 | WTI crude oil - long
economic hedges by derivative type [Line Items]
Notional contract volumes related to swap contracts serving as hedges	12,565,000
2013 | Ultra-low sulfur diesel - short
economic hedges by derivative type [Line Items]
Notional contract volumes related to swap contracts serving as hedges	11,125,000
2013 | Unleaded gasoline - short
economic hedges by derivative type [Line Items]
Notional contract volumes related to swap contracts serving as hedges	1,440,000
2014 | Natural gas - long
economic hedges by derivative type [Line Items]
Portion of future gas production being hedged	19,200,000
2014 | WTI crude oil - long
economic hedges by derivative type [Line Items]
Notional contract volumes related to swap contracts serving as hedges	365,000
2014 | Ultra-low sulfur diesel - short
economic hedges by derivative type [Line Items]
Notional contract volumes related to swap contracts serving as hedges	365,000
2014 | Unleaded gasoline - short
economic hedges by derivative type [Line Items]
Notional contract volumes related to swap contracts serving as hedges	0
2015 | Natural gas - long
economic hedges by derivative type [Line Items]
Portion of future gas production being hedged	19,200,000
2015 | WTI crude oil - long
economic hedges by derivative type [Line Items]
Notional contract volumes related to swap contracts serving as hedges	0
2015 | Ultra-low sulfur diesel - short
economic hedges by derivative type [Line Items]
Notional contract volumes related to swap contracts serving as hedges	0
2015 | Unleaded gasoline - short
economic hedges by derivative type [Line Items]
Notional contract volumes related to swap contracts serving as hedges	0
2016 | Natural gas - long
economic hedges by derivative type [Line Items]
Portion of future gas production being hedged	19,200,000
2016 | WTI crude oil - long
economic hedges by derivative type [Line Items]
Notional contract volumes related to swap contracts serving as hedges	0
2016 | Ultra-low sulfur diesel - short
economic hedges by derivative type [Line Items]
Notional contract volumes related to swap contracts serving as hedges	0
2016 | Unleaded gasoline - short
economic hedges by derivative type [Line Items]
Notional contract volumes related to swap contracts serving as hedges	0
2017 | Natural gas - long
economic hedges by derivative type [Line Items]
Portion of future gas production being hedged	19,200,000
2017 | WTI crude oil - long
economic hedges by derivative type [Line Items]
Notional contract volumes related to swap contracts serving as hedges	0
2017 | Ultra-low sulfur diesel - short
economic hedges by derivative type [Line Items]
Notional contract volumes related to swap contracts serving as hedges	0
2017 | Unleaded gasoline - short
economic hedges by derivative type [Line Items]
Notional contract volumes related to swap contracts serving as hedges	0