Fair Value Measurements (Details) - Recurring basis - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Inputs, Level 1
Fair value measurements
Cash and cash equivalents	$ 106.4	$ 113.0
Total assets at fair value	106.4	113.0
Fair Value, Inputs, Level 2
Fair value measurements
Cash and cash equivalents	0.0
Unrealized gains on natural gas derivatives		0.1
Total assets at fair value	0.0	0.1
Unrealized losses on natural gas derivatives	(27.9)	(3.6)
Total liabilities at fair value	(27.9)	(3.6)
Total
Fair value measurements
Cash and cash equivalents	106.4	113.0
Unrealized gains on natural gas derivatives		0.1
Total assets at fair value	106.4	113.1
Unrealized losses on natural gas derivatives	(27.9)	(3.6)
Total liabilities at fair value	$ (27.9)	$ (3.6)