Fair Value Measurements (Details) - Recurring basis - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Significant Observable Inputs
Fair value measurements
Cash Equivalents, at Fair Value	$ 0.0	$ 0.0
Unrealized gains on natural gas derivatives	0.0	9.0
Total assets at fair value	0.0	9.0
Unrealized losses on natural gas derivatives	(3.2)	(1.6)
Total liabilities at fair value	(3.2)	(1.6)
Total
Fair value measurements
Cash Equivalents, at Fair Value	80.5	32.0
Unrealized gains on natural gas derivatives	0.0	9.0
Total assets at fair value	80.5	41.0
Unrealized losses on natural gas derivatives	(3.2)	(1.6)
Total liabilities at fair value	(3.2)	(1.6)
Total | Quoted Market Prices in Active Markets (Level 1)
Fair value measurements
Cash Equivalents, at Fair Value	80.5	32.0
Unrealized gains on natural gas derivatives	0.0	0.0
Total assets at fair value	80.5	32.0
Unrealized losses on natural gas derivatives	$ 0.0	$ 0.0