Fair Value Measurement (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Changes in Level 3 fair value measurements
Fair value liability, beginning of period	$ 0				$ 0		$ (101.8)
Transfers out of Level 3	0	[1]	0	[1]	0	[1]	101.8	[1]
Realized and unrealized (gain) loss included in earnings	0		0		0		0
Settlements	0		0		0		0
Fair value liability, end of period	0		0		0		0
Level 3
Changes in Level 3 fair value measurements
Total unrealized (gain) loss included in earnings related to financial assets and liabilities still on the Condensed Balance Sheets at June 30, 2012 and 2011	$ 0		$ 0		$ 0		$ 0

[1] During the first quarter of 2011, the inputs used to value oil collars, natural gas collars and natural gas basis swaps were directly or indirectly observable, and these instruments were transferred to Level 2.