Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011	Dec. 31, 2010
Financial liabilities
Commodity derivative instruments - long-term	$ 0.1	$ 0
Fair Value, Measurements, Recurring [Member]
Financial Assets
Commodity derivative instruments - short-term	333.8	273.7
Commodity derivative instruments - long-term	115.6	123.5
Total financial assets	449.4	397.2
Financial liabilities
Commodity derivative instruments - short-term	0	1.3
Commodity derivative instruments - long-term	0.1	0
Total financial liabilities	0.1	1.3
Fair Value, Measurements, Recurring [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Financial Assets
Cash and cash equivalents	0	0
Financial liabilities
Checks outstanding in excess of cash balances	58.6	29.4
Long-term debt	1,673.5	1,679.4
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Financial Assets
Cash and cash equivalents	0	0
Financial liabilities
Checks outstanding in excess of cash balances	58.6	29.4
Long-term debt	1,770.8	1,754.9
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Financial Assets
Commodity derivative instruments - short-term	347.5	284.1
Commodity derivative instruments - long-term	115.8	123.5
Total financial assets	463.3	407.6
Financial liabilities
Commodity derivative instruments - short-term	11.6	11.7
Commodity derivative instruments - long-term	0.3	0
Total financial liabilities	11.9	11.7
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Financial Assets
Commodity derivative instruments - short-term	0	0
Commodity derivative instruments - long-term	0	0
Total financial assets	0	0
Financial liabilities
Commodity derivative instruments - short-term	2.1	0
Commodity derivative instruments - long-term	0	0
Total financial liabilities	2.1	0
Level 3 Unobservable input reconciliation [Roll Forward]
Balance at January 1,	(2.1)	0	23.2	36.3
Realized gains and losses	0	17.9
Unrealized gains and losses	(2.1)	(13.1)
Settlements	0	(17.9)
Balance at March 31,	(2.1)	0	23.2	36.3
Fair Value, Measurements, Recurring [Member] | Netting and Collateral [Member]
Financial Assets
Commodity derivative instruments - short-term	(13.7)	(10.4)
Commodity derivative instruments - long-term	(0.2)	0
Total financial assets	(13.9)	(10.4)
Financial liabilities
Commodity derivative instruments - short-term	(13.7)	(10.4)
Commodity derivative instruments - long-term	(0.2)	0
Total financial liabilities	$ (13.9)	$ (10.4)