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Fair Value of Financial Instruments - Additional Information (Detail) (USD $)
 7 Months Ended 12 Months Ended
Sep. 30, 2013
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2011
Jul. 31, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]          
Impairment of unproved oil and natural gas properties $ 10,300,000        
Impairment of proved oil and natural gas properties   54,300,000 0 0  
Percentage of female contractholder   50.00%      
Percentage of male contractholder   50.00%      
Fair Value Assets, level 1 to level 2 transfers     0   0
Net transfer In (Out) of Level 3 (a)   (53,000,000) [1] (731,800,000) [2] (17,700,000) [3]  
Transfers, assets, level 2 to 1 0 0     0
Transfers, liabilities, level 1 to 2 0 0     0
Transfers, liabilities, level 2 to 1 0 0     0
Net transfer level 3   0 30,900,000 [2]    
Fair Value, Measurements, Nonrecurring
          
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]          
Fair value of proved oil and gas properties 515,900,000 515,900,000      
Fair value of unproved oil and gas properties 36,500,000 36,500,000      
US Treasury and Government [Member]
          
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]          
Fair Value Assets, level 1 to level 2 transfers 79,300,000 79,300,000      
Asset-backed and Hybrid Securities
          
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]          
Fair Value Assets, unobservable inputs, transfers out of level 3   53,000,000      
Corporate and Non-Agency Residential Mortgage-Backed Securities
          
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]          
Fair Value Assets, unobservable inputs, transfers out of level 3     $ (731,800,000)    
[1] The net transfers in and out of Level 3 during the year ended September 30, 2013 were exclusively to or from Level 2.
[2] (a)The net transfers in and out of Level 3 during the year ended September 30, 2012 was exclusively to or from Level 2.
[3] (a)The net transfers in and out of Level 3 during the year ended September 30, 2011 was exclusively to or from Level 2.