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Debt - Additional Information (Detail) - USD ($)
3 Months Ended 12 Months Ended
Feb. 28, 2019
Feb. 24, 2017
Feb. 24, 2016
Jul. 06, 2015
Mar. 31, 2019
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2014
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Jan. 01, 2019
Aug. 01, 2017
Feb. 23, 2017
Debt Instrument [Line Items]                              
Debt instrument, covenant description                   The indenture governing the senior unsecured notes contains covenants that, among other things, limit the ability of the Company and its restricted subsidiaries to: (i) incur additional indebtedness, (ii) pay dividends on capital stock or redeem, repurchase or retire the Company’s capital stock or subordinated indebtedness, (iii) transfer or sell assets, (iv) make investments, (v) create certain liens, (vi) enter into agreements that restrict dividends or other payments to the Company from its restricted subsidiaries, (vii) consolidate, merge or transfer all or substantially all of the assets of the Company and its restricted subsidiaries, taken as a whole, (viii) engage in transactions with affiliates, and (ix) create unrestricted subsidiaries.          
Gain on early extinguishment of debt                       $ 14,489,000      
Outstanding letters of credit           $ 27,000,000.0       $ 27,000,000.0          
BRMR and Everest Merger Sub Inc. [Member] | Minimum [Member]                              
Debt Instrument [Line Items]                              
Ratio of total funded net debt to EBITDAX           400.00% 400.00% 400.00%              
Subsequent Event [Member]                              
Debt Instrument [Line Items]                              
Outstanding letters of credit                         $ 13,500,000    
Subsequent Event [Member] | BRMR and Everest Merger Sub Inc. [Member]                              
Debt Instrument [Line Items]                              
Line of credit facility extended maturity period 5 years                            
Scenario, Forecast [Member] | BRMR and Everest Merger Sub Inc. [Member] | Minimum [Member]                              
Debt Instrument [Line Items]                              
Ratio of total funded net debt to EBITDAX         400.00%                    
Revolving Credit Facility [Member]                              
Debt Instrument [Line Items]                              
Revolving credit facility                 $ 500,000,000            
Credit facility maturity year                 2018            
Applicable Margin     0.50%                        
Percentage of additional mortgage to be delivered     90.00%                        
Additional Period for the effectiveness of amendment     60 days                        
Borrowing base   $ 175,000,000       $ 225,000,000       225,000,000       $ 225,000,000 $ 125,000,000
Revolving credit facility, extended maturity month and year   2020-02                          
Outstanding borrowings           32,500,000       32,500,000          
Available capacity on the Revolving Credit Facility           $ 165,500,000       $ 165,500,000          
Percentage of company's properties and guarantees secured by mortgages           85.00%       85.00%          
Revolving Credit Facility [Member] | Minimum [Member]                              
Debt Instrument [Line Items]                              
Commitment fees on unused portion of revolving credit facility                   0.375%          
Revolving Credit Facility [Member] | Maximum [Member]                              
Debt Instrument [Line Items]                              
Commitment fees on unused portion of revolving credit facility                   0.50%          
Revolving Credit Facility [Member] | Subsequent Event [Member]                              
Debt Instrument [Line Items]                              
Borrowing base                         85,000,000    
Available capacity on the Revolving Credit Facility                         $ 244,000,000    
Revolving Credit Facility [Member] | Subsequent Event [Member] | BRMR and Everest Merger Sub Inc. [Member]                              
Debt Instrument [Line Items]                              
Borrowing base $ 375,000,000                            
8.875% Senior Unsecured Notes Due 2023 [Member]                              
Debt Instrument [Line Items]                              
Issuance date       Jul. 06, 2015                      
Debt instrument, outstanding principal balance amount       $ 550,000,000                      
Debt instrument interest rate       8.875%   8.875%       8.875%          
Debt instrument maturity year       2023                      
Notes issued percentage price       97.903%                      
Debt instrument, proceeds       $ 525,500,000                      
Amortization of deferred financing costs and debt discount                   $ 3,600,000 $ 3,400,000 3,300,000      
Principal amount outstanding                       39,500,000      
Gain on early extinguishment of debt                       14,500,000      
8.875% Senior Unsecured Notes Due 2023 [Member] | Level 2 [Member]                              
Debt Instrument [Line Items]                              
Fair value of senior unsecured notes           $ 437,900,000       $ 437,900,000          
8.875% Senior Unsecured Notes Due 2023 [Member] | Open Market [Member]                              
Debt Instrument [Line Items]                              
Debt instrument repurchase amount                       $ 23,400,000      
Senior PIK Notes [Member]                              
Debt Instrument [Line Items]                              
Debt instrument repurchase amount       $ 510,700,000