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Supplemental Gas and Oil Information - Unaudited Costs Incurred (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Property Acquisition [Abstract]      
Unproved $ 4.8 $ 11.9 $ 0.3
Proved 13.8 54.1 106.4
Exploration (capitalized and expensed) 0.7 1.6 0.0
Development 27.7 49.1 133.1
Total costs incurred 47.0 116.7 239.8
Cost incurred to develop proved undeveloped reserves $ 22.5 $ 28.9 $ 106.3