Fair Value Measurements (Details) - Recurring basis - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Quoted Market Prices in Active Markets (Level 1)
Fair value measurements
Cash and cash equivalents	$ 84.1	$ 106.4
Unrealized gains on natural gas derivatives	0.0
Total assets at fair value	84.1	106.4
Significant Other Observable Inputs (Level 2)
Fair value measurements
Cash and cash equivalents	0.0
Unrealized gains on natural gas derivatives	2.1
Total assets at fair value	2.1	0.0
Unrealized losses on natural gas derivatives	(5.0)	(27.9)
Total liabilities at fair value	(5.0)	(27.9)
Total
Fair value measurements
Cash and cash equivalents	84.1	106.4
Unrealized gains on natural gas derivatives	2.1
Total assets at fair value	86.2	106.4
Unrealized losses on natural gas derivatives	(5.0)	(27.9)
Total liabilities at fair value	$ (5.0)	$ (27.9)

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Cash
-URI http://asc.fasb.org/extlink&oid=6506951

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Cash Equivalents
-URI http://asc.fasb.org/extlink&oid=6507016

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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Fair value of financial and nonfinancial obligations.

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Fair value as of the balance sheet date of price risk derivative assets, which includes all such derivative instruments in hedging and nonhedging relationships that are recognized as assets on the balance sheet.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=49121117&loc=d3e13433-108611

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Fair value as of the balance sheet date of price risk derivative liabilities, which includes all such derivative instruments in hedging and nonhedging relationships that are recognized as liabilities on the balance sheet.

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