Fair Value Measurements (Details) - Recurring basis - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Quoted Market Prices in Active Markets (Level 1)
Fair value measurements
Cash and cash equivalents	$ 72.0	$ 113.0
Unrealized gains on natural gas derivatives	0.0
Total assets at fair value	72.0	113.0
Significant Other Observable Inputs (Level 2)
Fair value measurements
Unrealized gains on natural gas derivatives	3.1	0.1
Total assets at fair value	3.1	0.1
Unrealized losses on natural gas derivatives	(1.1)	(3.6)
Total liabilities at fair value	(1.1)	(3.6)
Total
Fair value measurements
Cash and cash equivalents	72.0	113.0
Unrealized gains on natural gas derivatives	3.1	0.1
Total assets at fair value	75.1	113.1
Unrealized losses on natural gas derivatives	(1.1)	(3.6)
Total liabilities at fair value	$ (1.1)	$ (3.6)