Fair Value Measurements (Details) - Recurring basis - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Significant Other Observable Inputs (Level 2)
Fair value measurements
Cash Equivalents, at Fair Value	$ 0.0	$ 0.0
Unrealized gains on natural gas derivatives	0.2	9.0
Total assets at fair value	0.2	9.0
Unrealized losses on natural gas derivatives	(2.6)	(1.6)
Total liabilities at fair value	(2.6)	(1.6)
Total
Fair value measurements
Cash Equivalents, at Fair Value	59.8	32.0
Unrealized gains on natural gas derivatives	0.2	9.0
Total assets at fair value	60.0	41.0
Unrealized losses on natural gas derivatives	(2.6)	(1.6)
Total liabilities at fair value	(2.6)	(1.6)
Total | Quoted Market Prices in Active Markets (Level 1)
Fair value measurements
Cash Equivalents, at Fair Value	59.8	32.0
Unrealized gains on natural gas derivatives	0.0	0.0
Total assets at fair value	59.8	32.0
Unrealized losses on natural gas derivatives	0.0	0.0
Total liabilities at fair value	$ 0.0	$ 0.0