Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified			3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended																				6 Months Ended										6 Months Ended	12 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2012 Unproved Properties [Member]	Jun. 30, 2012 Unproved Properties [Member]	Jun. 30, 2012 Proved Properties [Member]	Jun. 30, 2012 Proved Properties [Member]	Jun. 30, 2012 Fair Value, Measurements, Recurring [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member]	Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Commodity derivative [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Commodity derivative [Member]	Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Interest Rate Swap [Member]	Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Carrying Amount [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Carrying Amount [Member]	Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Level 1 [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Level 1 [Member]	Jun. 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]	Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Level 1 [Member] Interest Rate Swap [Member]	Jun. 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]	Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Level 2 [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Level 2 [Member]	Jun. 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]	Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Level 2 [Member] Interest Rate Swap [Member]	Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Level 3 [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Level 3 [Member]	Jun. 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]	Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Level 3 [Member] Interest Rate Swap [Member]	Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Netting Adjustments [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Netting Adjustments [Member]	Jun. 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]	Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Netting Adjustments [Member] Interest Rate Swap [Member]	Jun. 30, 2012 Fair Value, Measurements, Nonrecurring [Member] Proved Properties [Member]	Dec. 31, 2011 Fair Value, Measurements, Nonrecurring [Member] Proved Properties [Member]	Jun. 30, 2012 Fair Value, Measurements, Nonrecurring [Member] Level 3 [Member] Proved Properties [Member]	Dec. 31, 2011 Fair Value, Measurements, Nonrecurring [Member] Level 3 [Member] Proved Properties [Member]
Financial Assets
Commodity derivative instruments - short-term									$ 268.2	$ 273.7						$ 0	$ 0						$ 272.2	$ 284.1				$ 0	$ 0				$ (4.0)	$ (10.4)
Commodity derivative instruments - long-term									76.2	123.5						0	0						77.5	123.5				0	0				(1.3)	0
Total financial assets							344.4	397.2						0	0						349.7	407.6				0	0				(5.3)	(10.4)
Cash and Cash Equivalents, Fair Value Disclosure												146.4	0						146.4	0
Financial liabilities
Commodity derivative instruments - short-term									0	1.3	2.3					0	0	0					4.0	11.7	2.3				0				(4.0)	(10.4)	0
Commodity derivative instruments - long-term	2.4	0							0.4	0	2.0					0	0	0					1.7	0	2.0				0	0			(1.3)	0
Total financial liabilities							4.7	1.3						0	0						10.0	11.7				0	0				(5.3)	(10.4)
Checks outstanding in excess of cash balances												0	29.4						0	29.4
Long-term debt												1,866.6	1,679.4						1,967.2	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
Proved properties																																						5,058.6	4,833.2
Impairment			$ 6.1	$ 12.1	$ 48.9	$ 49.3																														$ 49.3	$ 195.5