Derivative Instruments (Details 3) (USD $)	12 Months Ended
	Dec. 31, 2012 counterparty	Dec. 31, 2011	Dec. 31, 2010
Location and amount of derivative instrument (gains) losses before income taxes recognized in the condensed statements of income
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, before Tax	$ 8,900,000	$ 61,800,000
Realized and unrealized (gain) loss on derivatives, net	(64,620,000)	(13,908,000)	31,847,000
Credit risk
Percentage of Derivative Value Held by Major Counterparties	74.00%
Derivative instruments, number of largest counterparties (in counterparties)	2
Sales Of Oil And Gas | Commodity Contract
Location and amount of derivative instrument (gains) losses before income taxes recognized in the condensed statements of income
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, before Tax	10,700,000	60,900,000	18,400,000
Realized And Unrealized Gain Loss On Derivatives Net | Commodity Contract
Location and amount of derivative instrument (gains) losses before income taxes recognized in the condensed statements of income
Realized and unrealized (gain) loss on derivatives, net	(64,600,000)	(13,900,000)	23,200,000
Realized And Unrealized Gain Loss On Derivatives Net | Interest rate contract
Location and amount of derivative instrument (gains) losses before income taxes recognized in the condensed statements of income
Realized and unrealized (gain) loss on derivatives, net	0	0	8,600,000
Interest Expense | Interest rate contract
Location and amount of derivative instrument (gains) losses before income taxes recognized in the condensed statements of income
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, before Tax	$ (1,800,000)	$ 800,000	$ 8,300,000