Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Financial liabilities
Derivative instruments - long-term	$ 3.6	$ 0
Level 3 Unobservable input reconciliation [Roll Forward]
Beginning Balance	0	0	36.3
Ending Balance	0	0	36.3
Impairment of Oil and Gas Properties	133.4	218.4	46.1
Proved properties, estimated fair values	71.9	157.2
Proved properties [Member]
Level 3 Unobservable input reconciliation [Roll Forward]
Impairment of Oil and Gas Properties	107.6	195.5	0.7
Fair Value, Measurements, Recurring [Member]
Financial Assets
Derivative Assets	192.8	397.2
Financial liabilities
Derivative Liabilities	6.2	1.3
Fair Value, Measurements, Recurring [Member] | Commodity Contract [Member]
Financial Assets
Derivative instruments - short-term	188.7	273.7
Derivative instruments - long-term	4.1	123.5
Financial liabilities
Derivative instruments - short-term	0	1.3
Derivative instruments - long-term	0	0
Fair Value, Measurements, Recurring [Member] | Interest Rate Swap [Member]
Financial liabilities
Derivative instruments - short-term	2.6
Derivative instruments - long-term	3.6
Fair Value, Measurements, Recurring [Member] | Carrying Amount [Member]
Financial liabilities
Checks outstanding in excess of cash balances	39.7	29.4
Long-term debt	3,206.9	1,679.4
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Financial Assets
Derivative Assets	0	0
Financial liabilities
Derivative Liabilities	0	0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Commodity Contract [Member]
Financial Assets
Derivative instruments - short-term	0	0
Derivative instruments - long-term	0	0
Financial liabilities
Derivative instruments - short-term	0	0
Derivative instruments - long-term	0	0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Interest Rate Swap [Member]
Financial liabilities
Derivative instruments - short-term	0
Derivative instruments - long-term	0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Financial liabilities
Checks outstanding in excess of cash balances	39.7	29.4
Long-term debt	3,420.7	1,754.9
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Financial Assets
Derivative Assets	193.9	407.6
Financial liabilities
Derivative Liabilities	7.3	11.7
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Commodity Contract [Member]
Financial Assets
Derivative instruments - short-term	189.7	284.1
Derivative instruments - long-term	4.2	123.5
Financial liabilities
Derivative instruments - short-term	1.0	11.7
Derivative instruments - long-term	0.1	0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Interest Rate Swap [Member]
Financial liabilities
Derivative instruments - short-term	2.6
Derivative instruments - long-term	3.6
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Financial Assets
Derivative Assets	0	0
Financial liabilities
Derivative Liabilities	0	0
Level 3 Unobservable input reconciliation [Roll Forward]
Realized gains and losses	0.6	25.3
Change in Unrealized Gain (Loss)	3.8	(36.3)
Settlements	(0.6)	(25.3)
Transfers out of Level 3	(3.8)	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Commodity Contract [Member]
Financial Assets
Derivative instruments - short-term	0	0
Derivative instruments - long-term	0	0
Financial liabilities
Derivative instruments - short-term	0	0
Derivative instruments - long-term	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Interest Rate Swap [Member]
Financial liabilities
Derivative instruments - short-term	0
Derivative instruments - long-term	0
Fair Value, Measurements, Recurring [Member] | Netting Adjustments [Member]
Financial Assets
Derivative Assets	(1.1)	(10.4)
Financial liabilities
Derivative Liabilities	(1.1)	(10.4)
Fair Value, Measurements, Recurring [Member] | Netting Adjustments [Member] | Commodity Contract [Member]
Financial Assets
Derivative instruments - short-term	(1.0)	(10.4)
Derivative instruments - long-term	(0.1)	0
Financial liabilities
Derivative instruments - short-term	(1.0)	(10.4)
Derivative instruments - long-term	(0.1)
Fair Value, Measurements, Recurring [Member] | Netting Adjustments [Member] | Interest Rate Swap [Member]
Financial liabilities
Derivative instruments - short-term	0
Derivative instruments - long-term	$ 0