Fair Value Measurements (Details) (Recurring basis, USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Total
Fair value measurements
Cash and cash equivalents	$ 172.7	$ 179.8
Total assets at fair value	172.7	179.8
Unrealized losses on natural gas derivatives	(9.5)	(12.0)
Total liabilities at fair value	(9.5)	(12.0)
Quoted Market Prices in Active Markets (Level 1)
Fair value measurements
Cash and cash equivalents	172.7	179.8
Total assets at fair value	172.7	179.8
Significant Other Observable Inputs (Level 2)
Fair value measurements
Unrealized losses on natural gas derivatives	(9.5)	(12.0)
Total liabilities at fair value	$ (9.5)	$ (12.0)