Long-Term Debt Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands							6 Months Ended
	Oct. 15, 2022	May 21, 2020	May 16, 2016	May 15, 2016	Oct. 03, 2012	Nov. 15, 2010	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Debt Instrument
Cash repayment of convertible notes							$ 115,000	$ 0
3.25% Convertible Senior Notes due 2016
Debt Instrument
Debt Instrument, Interest Rate, Stated Percentage							3.25%
3.25% Convertible Note, Conversion Price						$ 42.40
Debt Instrument, Issuance Date						Nov. 15, 2010
Debt Instrument, Maturity Date				May 15, 2016
7.75% Senior Notes due 2022
Debt Instrument
Debt Instrument, Interest Rate, Stated Percentage							7.75%
Debt Instrument, Issuance Date					Oct. 03, 2012
Debt Instrument, Maturity Date	Oct. 15, 2022
Revolving Credit Facility
Debt Instrument
Line of Credit Facility, Maximum Borrowing Capacity ($)							$ 1,000,000
Line of Credit Facility, Current Borrowing Capacity ($)							450,000
Line of Credit Facility, Increase (Decrease), Net							$ 250,000
Line of Credit Facility, Expiration Date		May 21, 2020
Debt Instrument, Covenant Description							The revolving credit facility contains covenants customary for agreements of this type, with the most restrictive being certain financial tests on a quarterly basis. The financial tests, as defined per the revolving credit facility, include requirements to: (a) maintain a minimum current ratio of 1.00 to 1.00 and (b) not exceed a maximum leverage ratio of 4.25 to 1.00.
Line of Credit Facility, Remaining Borrowing Capacity							$ 438,300
RNG Credit Facility [Member]
Debt Instrument
PDC Irrevocable Standby Letter of Credit ($)							$ 11,700
First Payment | 7.75% Senior Notes due 2022
Debt Instrument
Debt Instrument, Frequency of Periodic Payment							April 15
Second Payment | 7.75% Senior Notes due 2022
Debt Instrument
Debt Instrument, Frequency of Periodic Payment							October 15
Revolving Credit Facility
Debt Instrument
Long-term Line of Credit							$ 0		$ 37,000
Weighted Average Interest Rate									2.60%
Maximum Borrowing Base [Member] | Revolving Credit Facility
Debt Instrument
Line of Credit Facility, Current Borrowing Capacity ($)							$ 700,000
3.25% Convertible Senior Notes due 2016
Debt Instrument
Cash repayment of convertible notes			$ 115,000
Shares issued upon maturity of convertible notes			792,406
3.25% Convertible Note, Conversion Price			$ 42.40