Fair Value Measurements (Details) (Recurring basis, USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Total Fair Value
Fair value measurements
Cash and cash equivalents	$ 115.9	$ 86.9
Unrealized gains on natural gas derivatives	0.7	7.8
Total assets at fair value	116.6	94.7
Unrealized losses on natural gas derivatives	(0.5)
Total liabilities at fair value	(0.5)
Quoted Market Prices in Active Markets (Level 1)
Fair value measurements
Cash and cash equivalents	115.9	86.9
Total assets at fair value	115.9	86.9
Significant Other Observable Inputs (Level 2)
Fair value measurements
Unrealized gains on natural gas derivatives	0.7	7.8
Total assets at fair value	0.7	7.8
Unrealized losses on natural gas derivatives	(0.5)
Total liabilities at fair value	$ (0.5)