Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended		3 Months Ended	9 Months Ended																				9 Months Ended										9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Proved properties [Member]	Sep. 30, 2012 Proved properties [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Commodity derivative [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Commodity derivative [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Interest Rate Swap [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Carrying Amount [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Carrying Amount [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Level 1 [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Level 1 [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Level 1 [Member] Commodity derivative [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Level 1 [Member] Commodity derivative [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Level 1 [Member] Interest Rate Swap [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Level 1 [Member] Fair Value [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Level 1 [Member] Fair Value [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Level 2 [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Level 2 [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Level 2 [Member] Commodity derivative [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Level 2 [Member] Commodity derivative [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Level 2 [Member] Interest Rate Swap [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Level 3 [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Level 3 [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Level 3 [Member] Commodity derivative [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Level 3 [Member] Commodity derivative [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Level 3 [Member] Interest Rate Swap [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Netting Adjustments [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Netting Adjustments [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Netting Adjustments [Member] Commodity derivative [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Netting Adjustments [Member] Commodity derivative [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Netting Adjustments [Member] Interest Rate Swap [Member]	Sep. 30, 2012 Fair Value, Measurements, Nonrecurring [Member] Level 3 [Member] Proved properties [Member]
Financial Assets
Derivative instruments - short-term							$ 187.2	$ 273.7						$ 0	$ 0						$ 199.4	$ 284.1				$ 0	$ 0				$ (12.2)	$ (10.4)
Derivative instruments - long-term							35.2	123.5						0	0						38.2	123.5				0	0				(3.0)	0
Total financial assets					222.4	397.2						0	0						237.6	407.6				0	0				(15.2)	(10.4)
Financial liabilities
Derivative instruments - short-term							0.1	1.3	2.6					0	0	0					12.3	11.7	2.6			0	0	0			(12.2)	(10.4)	0
Derivative instruments - long-term	4.1	0					0	0	4.1					0	0	0					3.0	0	4.1			0	0	0			(3.0)	0	0
Total financial liabilities					6.8	1.3						0	0						22.0	11.7				0	0				(15.2)	(10.4)
Checks outstanding in excess of cash balances										27.5	29.4						27.5	29.4
Long-term debt										3,180.7	1,679.4						3,330.4	1,754.9
Level 3 Unobservable input reconciliation [Roll Forward]
Beginning Balance	0	0
Realized gains and losses																								0.6
Unrealized gains and losses																								3.8
Settlements																								(0.6)
Transfers out of Level 3																								(3.8)
Ending Balance	0	0
Impairment of Oil and Gas Properties	68.7		7.3	49.3																														49.3
Proved properties, estimated fair values																																		$ 36.7