Fair Value Measurements (Details) (Recurring basis, USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Total
Fair value measurements
Cash and cash equivalents	$ 113.9	$ 86.9
Unrealized gains on natural gas derivatives	4.9	7.8
Total assets at fair value	118.8	94.7
Quoted Market Prices in Active Markets (Level 1)
Fair value measurements
Cash and cash equivalents	113.9	86.9
Total assets at fair value	113.9	86.9
Significant Other Observable Inputs (Level 2)
Fair value measurements
Unrealized gains on natural gas derivatives	4.9	7.8
Total assets at fair value	$ 4.9	$ 7.8