Fair Value Measurements (Details 2) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Assets and liabilities measured at fair value on a recurring basis
Cash and cash equivalents	$ 2,286.2	$ 2,274.9
Restricted cash	111.4
Asset retirement obligation funds	200.8	200.8
Recurring basis | Quoted Prices in Active Markets (Level 1)
Assets and liabilities measured at fair value on a recurring basis
Cash and cash equivalents	2,286.2	2,274.9
Restricted cash	111.4
Asset retirement obligation funds	200.8	200.8
Nonqualified employee benefit trusts		22.0
Total assets at fair value	2,598.4	2,497.7
Recurring basis | Significant Other Observable Inputs (Level 2)
Assets and liabilities measured at fair value on a recurring basis
Unrealized gains on derivative instruments	24.2	17.3
Total assets at fair value	24.2	17.3
Unrealized losses on derivative instruments	9.0	5.6
Total liabilities at fair value	9.0	5.6
Recurring basis | Significant Unobservable Inputs (Level 3)
Assets and liabilities measured at fair value on a recurring basis
Investments in auction rate securities		26.0
Total assets at fair value		26.0
Recurring basis | Total Fair Value
Assets and liabilities measured at fair value on a recurring basis
Cash and cash equivalents	2,286.2	2,274.9
Restricted cash	111.4
Unrealized gains on derivative instruments	24.2	17.3
Asset retirement obligation funds	200.8	200.8
Investments in auction rate securities		26.0
Nonqualified employee benefit trusts		22.0
Total assets at fair value	2,622.6	2,541.0
Unrealized losses on derivative instruments	9.0	5.6
Total liabilities at fair value	$ 9.0	$ 5.6