Fair Value of Financial Instruments - Changes in Fair Value of Financial Instruments (Detail) - USD ($)		12 Months Ended
		Sep. 30, 2016		Sep. 30, 2015		Sep. 30, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Assets, level 1 to level 2 transfers		$ 0				$ 0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period		91,600,000		$ 116,700,000		86,300,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		(1,400,000)		(10,500,000)		3,300,000
Total Gains (Losses) Included in AOCI						0
Purchases		36,200,000		30,400,000		41,300,000
Sales		(48,000,000)		(14,100,000)		(700,000)
Settlements		(5,500,000)		(50,000,000)		(13,500,000)
Net transfer In (Out) of Level 3 (a)		5,200,000	[1]	19,100,000	[2]	0
Balance at End of Period		78,100,000		91,600,000		116,700,000
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period		771,500,000		302,100,000		471,300,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings		48,400,000		15,800,000		30,000,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Other Comprehensive Income (Loss)		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases		0		444,200,000		150,600,000
Sales		0		0		0
Settlements		(56,900,000)		9,400,000		(349,800,000)
Net transfer In (Out) of Level 3		0	[1]	0	[2]	0
Balance at End of Period		763,000,000		771,500,000		302,100,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3				0		30,900,000	[1]
Embedded Derivatives Included In Contractholder Funds [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period				150,800,000		140,500,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings						10,300,000
Balance at End of Period						150,800,000
Front Street Future Policyholder Benefits Reserve [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period		629,200,000		151,300,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings		59,500,000		24,300,000		7,000,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases				444,200,000		150,600,000
Settlements		56,900,000		9,400,000		(6,300,000)
Balance at End of Period		631,800,000		629,200,000		151,300,000
Equity Conversion Features Of Preferred Stock [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period				0		330,800,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings						12,700,000
Sales						0
Balance at End of Period						0
Available-for-sale embedded derivatives
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Other Comprehensive Income (Loss)				0		0
FIA embedded derivatives, included in contractholder funds
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period		142,300,000		150,800,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings		(11,100,000)		(8,500,000)
Balance at End of Period		131,200,000		142,300,000		150,800,000
Equity Conversion Features Of Preferred Stock [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total Gains (Losses) Included in AOCI						0
Net transfer In (Out) of Level 3 (a)						0
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases						0
Settlements						(343,500,000)
Corporate Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period		14,100,000		16,300,000		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		(500,000)		(2,200,000)		0
Total Gains (Losses) Included in AOCI						0
Purchases		0				16,300,000
Sales		(13,600,000)				0
Settlements		0				0
Net transfer In (Out) of Level 3 (a)						0
Balance at End of Period				14,100,000		16,300,000
Equity Securities Classified As Held For Trading [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period				0		10,700,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings						1,300,000
Total Gains (Losses) Included in AOCI						0
Purchases						1,500,000
Sales						0
Settlements						(13,500,000)
Net transfer In (Out) of Level 3 (a)						0
Balance at End of Period						0
Other invested assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period		2,800,000		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		2,700,000		(16,300,000)
Total Gains (Losses) Included in AOCI		0		0
Purchases		0		0
Sales		0		0
Settlements		(5,500,000)		0
Net transfer In (Out) of Level 3 (a)		0	[1]	19,100,000	[2]
Balance at End of Period		0		2,800,000		0
Funds withheld for reinsurance liability [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period		74,700,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		(3,600,000)
Total Gains (Losses) Included in AOCI		0
Purchases		36,200,000
Sales		(34,400,000)
Settlements		0
Net transfer In (Out) of Level 3 (a)	[1]	5,200,000
Balance at End of Period		78,100,000		74,700,000
Embedded Derivatives Included In Contractholder Funds [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total Gains (Losses) Included in AOCI						0
Net transfer In (Out) of Level 3 (a)						0
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases						0
Sales						0
Settlements						0
Funds Withheld Receivables [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period		$ 74,700,000		58,900,000		34,600,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings				(500,000)		1,500,000
Purchases				30,400,000		23,500,000
Sales				(14,100,000)		(700,000)
Balance at End of Period				74,700,000		58,900,000
Contingent Purchase Price Reduction Receivable [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period				41,500,000		41,000,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings				8,500,000		500,000
Total Gains (Losses) Included in AOCI						0
Purchases						0
Sales						0
Settlements				$ (50,000,000)		0
Net transfer In (Out) of Level 3 (a)						0
Balance at End of Period						$ 41,500,000

[1] (a) During Fiscal 2016, the net transfer to Level 3 was exclusively from Level 2. The transfers into Level 3 were related to changes in the primary pricing source and changes in the observability of external information used in determining the fair value.

[2] (a) During Fiscal 2015, the net transfer to Level 3 was related to a loan receivable previously classified as a related party loan.