Commitments and Contingencies (Other Commitments) (Details) MBbls in Millions, $ in Millions, Mcfe in Billions	3 Months Ended	12 Months Ended
	Mar. 31, 2016 USD ($) Mcfe item	Dec. 31, 2014 Mcfe MBbls	Dec. 31, 2015 USD ($)	Aug. 10, 2015 USD ($)	Jul. 15, 2015 USD ($)
Commitments and Contingencies
Outstanding balance	$ 2,800.0
Nordheim
Other Commitments
Term of agreement		10 years
Minimum volume required (in Bcfe) | Mcfe		88.5
Minimum volume required (in MBbl) | MBbls		5,150
Properties in Core Operating Areas
Other Commitments
Minimum volume required (in Bcfe) | Mcfe	6.8
HPIP Gonzales Holdings, LLC
Drilling and Completion
Number of wells required to be drilled annually | item	1
Revolving Credit Facility
Commitments and Contingencies
Outstanding balance	$ 902.0
Term Loan Facility
Commitments and Contingencies
Outstanding balance	700.0				$ 650.0
2019 Senior Notes
Commitments and Contingencies
Outstanding balance	577.9
2020 Senior Notes
Commitments and Contingencies
Outstanding balance	222.1
Liabilities Subject to Compromise | Drilling and Other Services
Drilling and Completion
Amount allowed by court to be rejected				$ 29.0
Liabilities Subject to Compromise | Nordheim
Other Commitments
Minimum delivery commitment shortfall penalty			$ 3.0
Liabilities Subject to Compromise | Properties in Core Operating Areas
Other Commitments
Minimum delivery commitment shortfall penalty	1.6		$ 1.3
Liabilities Subject to Compromise | Office and Equipment Leases
Drilling and Completion
Amount allowed by court to be rejected	$ 3.6