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Long-Term Debt (Narrative) (Details) |  | 1 Months Ended | 3 Months Ended | 9 Months Ended |  |  |  |  |  |  |  |  |
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Dec. 27, 2013 USD ($) | Jan. 31, 2016 | Sep. 30, 2016 USD ($) | Sep. 30, 2015 USD ($) | Sep. 30, 2016 USD ($) | Sep. 30, 2015 USD ($) | Feb. 19, 2016 USD ($) | Feb. 18, 2016 USD ($) | Dec. 31, 2015 USD ($) | Jun. 04, 2015 USD ($) | Apr. 21, 2015 USD ($) | Aug. 18, 2014 USD ($) | Dec. 21, 2012 USD ($) | Oct. 17, 2012 USD ($) | Mar. 31, 2011 USD ($)
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest cost capitalized, undeveloped properties |  |  |  | $ 4,700,000 | $ 3,600,000 | $ 7,700,000 | $ 12,000,000 |  |  |  |  |  |  |  |  |  |
Interest capitalized |  |  |  |  |  | $ 8,920,000 | $ 12,041,000 |  |  |  |  |  |  |  |  |  |
Debt covenant ratio for EBITDAX |  |  |  | 3.00 |  | 3.00 |  |  |  |  |  |  |  |  |  |  |
Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt covenant ratio for future EBITDAX |  |  |  | 4.00 |  | 4.00 |  |  |  |  |  |  |  |  |  |  |
Building loan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest capitalized |  |  |  | $ 500,000 |  | $ 1,200,000 |  |  |  |  |  |  |  |  |  |  |
Building loan outstanding amount of building loan refinanced |  |  |  |  |  |  |  |  |  | $ 1,653,000 | [1] |  |  |  |  |  | $ 2,400,000
Loan, periodic payment |  |  |  |  |  | 20,000 |  |  |  |  |  |  |  |  |  |  |
7.75% Senior Notes due 2020 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Building loan outstanding amount of building loan refinanced | [2] |  |  | $ 600,000,000 |  | $ 600,000,000 |  |  |  | $ 600,000,000 |  |  |  |  |  |  |
Stated interest rate, percent |  |  |  | 7.75% |  | 7.75% |  |  |  | 7.75% |  |  |  |  |  |  |
Debt issued |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 300,000,000.0 | $ 50,000,000.0 | $ 250,000,000.0 |
Redemption of principal amount plus aggregate net proceeds, percent |  |  |  | 100.00% |  | 100.00% |  |  |  |  |  |  |  |  |  |  |
7.75% Senior Notes due 2020 | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of notes required to be outstanding for redemption, percent |  |  |  | 65.00% |  | 65.00% |  |  |  |  |  |  |  |  |  |  |
7.75% Senior Notes due 2020 | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Redemption of principal amount plus aggregate net proceeds, percent |  |  |  | 35.00% |  | 35.00% |  |  |  |  |  |  |  |  |  |  |
Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate, percent |  |  |  |  |  |  |  |  |  |  |  |  |  | 7.75% |  |  |
Debt instrument, amount |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 600,000,000 |  |  |
Discount issue price, price, percent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 98.534% |
Unamortized discount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 3,700,000 |
Effective interest rate, percent |  |  |  |  |  |  |  |  |  |  |  |  |  | 6.561% | 7.531% | 8.00% |
Premium issue price, percent |  |  |  |  |  |  |  |  |  |  |  |  |  | 106.00% | 101.00% |  |
Unamortized premium |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 18,000,000 | $ 500,000 |  |
Construction Loans |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Building loan outstanding amount of building loan refinanced | [3] |  |  | $ 16,499,000 |  | $ 16,499,000 |  |  |  | $ 0 |  |  |  |  |  |  |
Net unamortized debt issuance costs |  |  |  | $ 100,000 |  | $ 100,000 |  |  |  |  |  |  |  |  |  |  |
Line of credit facility, required minimum down payment, percent |  |  | 30.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fifth Amended And Restated Credit Agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Borrowing capacity |  |  |  |  |  |  |  | $ 700,000,000.0 |  |  |  |  |  |  |  |  |
Refinanced Building Loan | Building loan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate, percent |  |  |  | 4.00% |  | 4.00% |  |  |  |  |  |  |  |  |  |  |
Notes Due 2023 | Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate, percent |  |  |  |  |  |  |  |  |  |  |  |  | 6.625% |  |  |  |
Debt instrument, amount |  |  |  |  |  |  |  |  |  |  |  |  | $ 350,000,000.0 |  |  |  |
6.625% Senior Notes | Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Building loan outstanding amount of building loan refinanced |  |  |  |  |  |  |  |  |  |  |  |  | $ 343,600,000 |  |  |  |
Stated interest rate, percent |  |  |  |  |  |  |  |  |  |  |  |  | 6.625% |  |  |  |
October Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unamortized discount |  |  |  | $ 2,100,000 |  | $ 2,100,000 |  |  |  |  |  |  |  |  |  |  |
Net unamortized debt issuance costs |  |  |  | 4,400,000 |  | 4,400,000 |  |  |  |  |  |  |  |  |  |  |
December Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unamortized premium |  |  |  | 300,000 |  | 300,000 |  |  |  |  |  |  |  |  |  |  |
Net unamortized debt issuance costs |  |  |  | 1,000,000 |  | 1,000,000 |  |  |  |  |  |  |  |  |  |  |
August Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unamortized premium |  |  |  | 12,600,000 |  | 12,600,000 |  |  |  |  |  |  |  |  |  |  |
Net unamortized debt issuance costs |  |  |  | 4,300,000 |  | 4,300,000 |  |  |  |  |  |  |  |  |  |  |
April Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net unamortized debt issuance costs |  |  |  | 6,200,000 |  | 6,200,000 |  |  |  |  |  |  |  |  |  |  |
Letter of Credit | Fifth Amended And Restated Credit Agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding |  |  |  | 207,000,000 |  | 207,000,000 |  |  |  |  |  |  |  |  |  |  |
Remaining borrowing capacity |  |  |  | $ 493,000,000 |  | $ 493,000,000 |  |  |  |  |  |  |  |  |  |  |
Nova Scotia, Amegy, KeyBank |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revolving credit facility |  | $ 1,500,000,000.0 |  |  |  |  |  | $ 1,350,000,000.00 | $ 1,200,000,000.0 |  |  |  |  |  |  |  |
Credit facility outstanding |  |  |  |  |  |  |  |  | $ 950,000,000 |  |  |  |  |  |  |  |
Debt instrument, description of variable rate basis |  |  |  |  |  | LIBOR01 |  |  |  |  |  |  |  |  |  |  |
Nova Scotia, Amegy, KeyBank | Base Rate Loans |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Applicable rate, minimum, percent |  | 0.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Applicable rate, maximum, percent |  | 1.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Nova Scotia, Amegy, KeyBank | Base Rate Loans | Federal Funds Rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis spread, percent |  | 0.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Nova Scotia, Amegy, KeyBank | Base Rate Loans | Eurodollar |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis spread, percent |  | 1.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Nova Scotia, Amegy, KeyBank | Euro Dollar Loans |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Applicable rate, minimum, percent |  | 1.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Applicable rate, maximum, percent |  | 2.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Nova Scotia, Amegy, KeyBank | Letter of Credit | Second Amendment of Restated Credit Agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disposition costs, maximum expenses allowed |  |  |  |  |  | $ 3,000,000 |  |  |  |  |  |  |  |  |  |  |
InterBank | Line of Credit | Construction Loans |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revolving credit facility |  |  |  |  |  |  |  |  |  |  |  | $ 24,500,000.0 |  |  |  |  |
Stated interest rate, percent |  |  |  |  |  |  |  |  |  |  |  | 4.50% |  |  |  |  |
[1] | In March 2011, the Company refinanced the $2.4 million then outstanding under its previous building loan for the office building it occupies in Oklahoma City, Oklahoma. This loan agreement, as subsequently amended, bore interest at the rate of 4.00% per annum, required monthly interest and principal payments of approximately $20,000, was collateralized by the Oklahoma City office building and associated land and had a maturity date of December 2018. The Company paid the balance of the loan in full in February 2016.
[2] | On October 17, 2012, the Company issued $250.0 million in aggregate principal amount of senior unsecured notes due 2020 (the "October Notes") under an indenture among the Company, its subsidiary guarantors and Wells Fargo Bank, National Association, as the trustee (the "senior note indenture"). On December 21, 2012, the Company issued an additional $50.0 million in aggregate principal amount of senior unsecured notes due 2020 (the "December Notes") as additional securities under the senior note indenture. On August 18, 2014, the Company issued an additional $300.0 million in aggregate principal amount of senior unsecured notes due 2020 (the "August Notes"). The August Notes were issued as additional securities under the senior note indenture. The October Notes, December Notes and the August Notes are collectively referred to as the "2020 Notes."In connection with the issuance of the 2020 Notes, the Company and the subsidiary guarantors entered into a registration rights agreement with the initial purchasers, pursuant to which the Company and the subsidiary guarantors agreed to file a registration statement with respect to an offering to exchange the 2020 Notes for a new issue of substantially identical debt securities registered under the Securities Act. The exchange offer for the October Notes and the December Notes was completed in October 2013 and the exchange offer for the August Note was completed in March 2015. Under the senior note indenture relating to the 2020 Notes, interest on the 2020 Notes accrues at a rate of 7.75% per annum on the outstanding principal amount from October 17, 2012, payable semi-annually on May 1 and November 1 of each year, commencing on May 1, 2013. The 2020 Notes are the Company's senior unsecured obligations and rank equally in the right of payment with all of the Company's other senior indebtedness and senior in right of payment to any future subordinated indebtedness. All of the Company's existing and future restricted subsidiaries that guarantee the Company's secured revolving credit facility or certain other debt guarantee the 2020 Notes; provided, however, that the 2020 Notes are not guaranteed by Grizzly Holdings, Inc. and will not be guaranteed by any of the Company's future unrestricted subsidiaries. The Company may redeem some or all of the 2020 Notes at any time on or after November 1, 2016, at the redemption prices listed in the senior note indenture. Prior to November 1, 2016, the Company may redeem the 2020 Notes at a price equal to 100% of the principal amount plus a “make-whole” premium. In addition, prior to November 1, 2015, the Company may redeem up to 35% of the aggregate principal amount of the 2020 Notes with the net proceeds of certain equity offerings, provided that at least 65% of the aggregate principal amount of the 2020 Notes initially issued remains outstanding immediately after such redemption. See Note 16, Subsequent Events - "Tender Offer and Redemption - Existing 2020 Notes."
[3] | On June 4, 2015, the Company entered into a construction loan agreement (the "Construction Loan") with InterBank for the construction of a new corporate headquarters in Oklahoma City. The Construction Loan allows for a maximum principal amount of $24.5 million and requires the Company to fund 30% of the estimated cost of the construction before any funds can be drawn, which occurred in January 2016. Interest accrues daily on the outstanding principal balance at a fixed rate of 4.50% per annum and is payable on the last day of the month through May 31, 2017. Monthly interest and principal payments are due beginning June 30, 2017, with the final payment due June 4, 2025. At September 30, 2016, the total borrowings under the Construction Loan were approximately $16.5 million.

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- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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