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Fair Value of Financial Instruments - Additional Information (Detail) - USD ($)
 9 Months Ended 12 Months Ended
Jun. 30, 2015
Jun. 30, 2013
 [1]
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]          
Impairment of proved oil and natural gas properties     $ 485,100,000 $ 81,000,000 $ 54,300,000
Fair Value Assets, level 1 to level 2 transfers     0 0  
Net transfer In (Out) of Level 3 (a) $ (34,500,000) $ (53,000,000) (15,400,000) [2] (23,900,000) [3]  
Transfers, assets, level 2 to 1     0 0  
Transfers, liabilities, level 1 to 2     0 0  
Transfers, liabilities, level 2 to 1     0 0  
Net transfer level 3     0 30,900,000 [2]  
United States Government and government agencies and authorities          
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]          
Fair Value Assets, level 1 to level 2 transfers         79,300,000
Compass Production Partners [Member]          
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]          
Impairment of Unproved Oil and Gas Properties     $ 21,900,000 $ 6,100,000 $ 10,300,000
[1] (a) The net transfers out of Level 3 during Fiscal 2013 were exclusively to Level 2.
[2] (a) During Fiscal 2015, the net transfers out of Level 3 were exclusively to Level 2 and the net transfer to Level 3 was related to a loan receivable previously eliminated upon consolidation.
[3] (a) The net transfers in and out of Level 3 during Fiscal 2014 were exclusively to or from Level 2.