Fair Value of Financial Instruments - Additional Information (Detail) (USD $)	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013		Sep. 30, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Document Period End Date	Sep. 30, 2014
Impairment of proved oil and natural gas properties	$ 81,000,000	$ 54,300,000		$ 0
Percentage of female contractholder	50.00%
Percentage of male contractholder	50.00%
Fair Value Assets, level 1 to level 2 transfers	0	0		0
Net transfer In (Out) of Level 3 (a)	(23,900,000)	(53,000,000)		(731,800,000)	[1]
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
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	23,900,000
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				(53,000,000)
Commercial Mortgage Loan [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Receivable, Fair Value Disclosure	$ 135,400,000

[1] (a)The net transfers in and out of Level 3 during Fiscal 2012 was exclusively to or from Level 2.

[2] (a)The net transfers in and out of Level 3 during Fiscal 2013 was exclusively to or from Level 2.