Fair Value Measurements (Details 2) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2014 item	Dec. 31, 2013
Assets and liabilities measured at fair value on a recurring basis
Cash and cash equivalents	$ 2,194.1	$ 1,710.8
Restricted cash	645.0	154.0
Number of cash accounts restricted	2
Amount deposited into restricted cash arrangement	454.8
Replacement Period	6 months
Amount expected to be spent in Replacement Period	304
Carrying value of long-term debt, including current portion	4,592.4	3,098.1
Fair value of long-term debt, including current portion	5,012.4	3,276.7
Recurring basis | Total Fair Value
Assets and liabilities measured at fair value on a recurring basis
Cash and cash equivalents	2,194.1	1,710.8
Restricted cash	645.0	154.0
Unrealized gains on derivative instruments	15.9	74.3
Asset retirement obligation funds		203.7
Total assets at fair value	2,855.0	2,142.8
Unrealized losses on derivative instruments	3.2	0.2
Total liabilities at fair value	3.2	0.2
Recurring basis | Quoted Prices in Active Markets (Level 1)
Assets and liabilities measured at fair value on a recurring basis
Cash and cash equivalents	2,194.1	1,710.8
Restricted cash	645.0	154.0
Asset retirement obligation funds		203.7
Total assets at fair value	2,839.1	2,068.5
Recurring basis | Significant Other Observable Inputs (Level 2)
Assets and liabilities measured at fair value on a recurring basis
Unrealized gains on derivative instruments	15.9	74.3
Total assets at fair value	15.9	74.3
Unrealized losses on derivative instruments	3.2	0.2
Total liabilities at fair value	$ 3.2	$ 0.2