Fair Value of Financial Instruments - Additional Information (Detail) (USD $)	3 Months Ended				6 Months Ended
	Mar. 31, 2014		Mar. 31, 2013		Mar. 31, 2014		Mar. 31, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers Between Asset Classification	$ 6,000,000				$ 6,000,000
Percentage of male contractholder					50.00%
Percentage of female contractholder					50.00%
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	0		79,300,000		0		79,300,000
Net transfer In (Out) of Level 3 (a)	$ (5,000,000)	[1]	$ (38,900,000)	[2]	$ (5,000,000)	[1]	$ (53,000,000)	[2]

[1] This includes a $6.0 transfer to asset-backed securities from commercial mortgage-backed securities, the remaining transfers were from level 3 to level 2.

[2] (a)The net transfers in and out of Level 3 during the six months ended March 31, 2013 were exclusively to or from Level 2.