Fair Value Measurements (Details) (Recurring basis, USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Total
Fair value measurements
Cash and cash equivalents	$ 290.6	$ 149.4
Unrealized gains on natural gas derivatives	3.1	0.2
Total assets at fair value	293.7	149.6
Unrealized losses on natural gas derivatives		1.0
Total liabilities at fair value		1.0
Quoted Market Prices in Active Markets (Level 1)
Fair value measurements
Cash and cash equivalents	290.6	149.4
Total assets at fair value	290.6	149.4
Significant Other Observable Inputs (Level 2)
Fair value measurements
Unrealized gains on natural gas derivatives	3.1	0.2
Total assets at fair value	3.1	0.2
Unrealized losses on natural gas derivatives		1.0
Total liabilities at fair value		$ 1.0