Fair Value Measurements (Details 2) (USD $) In Millions, unless otherwise specified			3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2012 Minimum day	Mar. 31, 2012 Maximum day	Mar. 31, 2012 Recurring basis Maximum Y	Mar. 31, 2012 Recurring basis Quoted Prices in Active Markets (Level 1)	Dec. 31, 2011 Recurring basis Quoted Prices in Active Markets (Level 1)	Mar. 31, 2012 Recurring basis Significant Other Observable Inputs (Level 2)	Dec. 31, 2011 Recurring basis Significant Other Observable Inputs (Level 2)	Mar. 31, 2012 Recurring basis Significant Unobservable Inputs (Level 3)	Dec. 31, 2011 Recurring basis Significant Unobservable Inputs (Level 3)	Mar. 31, 2012 Recurring basis Total Fair Value	Dec. 31, 2011 Recurring basis Total Fair Value
Assets and liabilities measured at fair value on a recurring basis
Cash and cash equivalents	$ 1,713.3	$ 1,207.0				$ 1,713.3	$ 1,207.0					$ 1,713.3	$ 1,207.0
Unrealized gains on natural gas derivatives								0.2	0.5			0.2	0.5
Asset retirement obligation funds	147.6	145.4				147.6	145.4					147.6	145.4
Investments in auction rate securities	71.1	70.9								71.1	70.9	71.1	70.9
Nonqualified employee benefit trusts						21.7	20.2					21.7	20.2
Total assets at fair value						1,882.6	1,372.6	0.2	0.5	71.1	70.9	1,953.9	1,444.0
Unrealized losses on natural gas derivatives								130.3	74.7			130.3	74.7
Total liabilities at fair value								$ 130.3	$ 74.7			$ 130.3	$ 74.7
Auction rate securities interest rate reset period (in days)			7	35
Maturity period of auction rate securities (in years)					36