Organization and Basis of Presentation (Details) - USD ($)	3 Months Ended		9 Months Ended
	Mar. 31, 2016	Jun. 30, 2015	Sep. 30, 2016	Apr. 13, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash			$ 447,925,000		$ 1,272,000	$ 701,000	$ 1,852,000
Current borrowing capacity			1,050,000,000		1,400,000,000
Proceeds from issuance of common stock net of offering expenses	$ 381,100,000	$ 398,600,000
Long-term debt carrying value			554,000,000		776,500,000
One-time Termination Benefits
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Restructuring charges incurred as a result of workforce reduction			5,000,000
Common Stock
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Shares issued through public equity offering (in shares)	18,170,000	5,700,000
Credit facility
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Current borrowing capacity			1,050,000,000	$ 1,050,000,000
Long-term debt carrying value			0		$ 222,500,000
Long-term note agreements
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Long-term debt carrying value			554,000,000
Permian and San Juan Basin | Disposal group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Amount of consideration received			$ 552,000,000