Fair Value of Financial Instruments - Changes in Fair Value of Financial Instruments (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]
Document Period End Date					Mar. 31, 2014
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	$ 0		$ 79,300,000		$ 0		$ 79,300,000
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	709,500,000		313,500,000		523,500,000		206,000,000
Total Gains (Losses) Included in Earnings			(100,000)		600,000		(300,000)
Total Gains (Losses) Included in AOCI	11,300,000		3,300,000		4,600,000		1,200,000
Purchases	41,700,000		154,200,000		234,300,000		288,400,000
Sales							(9,600,000)
Settlements	(1,900,000)		(13,000,000)		(2,400,000)		(13,700,000)
Net transfer In (Out) of Level 3 (a)	(5,000,000)	[1]	(38,900,000)	[2]	(5,000,000)	[1]	(53,000,000)	[2]
Balance at End of Period	755,600,000		419,000,000		755,600,000		419,000,000
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	2,172,600,000		1,680,100,000		1,875,200,000		1,782,800,000
Total Gains (Losses) Included in Earnings	81,200,000		162,200,000		228,000,000		59,500,000
Total Gains (Losses) Included in AOCI	0				0
Purchases	0				150,600,000
Sales	0				0
Settlements	19,300,000				19,300,000
Net transfer In (Out) of Level 3	0			[2]	0	[1]		[2]
Balance at End of Period	2,234,500,000		1,842,300,000		2,234,500,000		1,842,300,000
FIA 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	1,644,700,000		1,517,000,000		1,544,400,000		1,550,800,000
Total Gains (Losses) Included in Earnings	74,000,000		122,600,000		174,300,000		88,800,000
Total Gains (Losses) Included in AOCI
Purchases
Sales
Settlements
Net transfer In (Out) of Level 3		[1]		[2]		[1]		[2]
Balance at End of Period	1,718,700,000		1,639,600,000		1,718,700,000		1,639,600,000
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	149,900,000				0
Total Gains (Losses) Included in Earnings	3,700,000				3,000,000
Total Gains (Losses) Included in AOCI					0
Purchases					150,600,000
Sales					0
Settlements	(2,600,000)				2,600,000
Net transfer In (Out) of Level 3		[1]			0	[1]
Balance at End of Period	151,000,000				151,000,000
Equity conversion feature of preferred stock [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	378,000,000		163,100,000		330,800,000		232,000,000
Total Gains (Losses) Included in Earnings	3,500,000		39,600,000		50,700,000		(29,300,000)
Total Gains (Losses) Included in AOCI
Purchases
Sales
Settlements	(16,700,000)				(16,700,000)
Net transfer In (Out) of Level 3		[1]		[2]		[1]		[2]
Balance at End of Period	364,800,000		202,700,000		364,800,000		202,700,000
Asset-backed securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	9,800,000		5,300,000		5,000,000		15,900,000
Total Gains (Losses) Included in Earnings	0				0
Total Gains (Losses) Included in AOCI	(100,000)				(300,000)		(100,000)
Purchases	0				5,000,000
Sales
Settlements	0				0
Net transfer In (Out) of Level 3 (a)	1,000,000	[1]		[2]	1,000,000	[1]	(10,500,000)	[2]
Balance at End of Period	10,700,000		5,300,000		10,700,000		5,300,000
ABS and Hybrids [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net transfer In (Out) of Level 3 (a)							0	[2]
Commercial mortgage-backed securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	6,000,000		6,100,000		5,700,000		5,000,000
Total Gains (Losses) Included in Earnings
Total Gains (Losses) Included in AOCI			100,000		300,000		200,000
Purchases							1,000,000
Sales
Settlements	0				0
Net transfer In (Out) of Level 3 (a)	(6,000,000)	[1]		[2]	(6,000,000)	[1]		[2]
Balance at End of Period			6,200,000				6,200,000
Corporates [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	607,100,000		256,100,000		461,100,000		135,300,000
Total Gains (Losses) Included in Earnings	0		(100,000)		0		(300,000)
Total Gains (Losses) Included in AOCI	10,100,000		3,200,000		4,000,000		1,200,000
Purchases	41,700,000		144,200,000		194,300,000		277,400,000
Sales	0				0		(9,600,000)
Settlements	(1,900,000)		(13,000,000)		(2,400,000)		(13,700,000)
Net transfer In (Out) of Level 3 (a)		[1]	(33,900,000)	[2]		[1]	(33,800,000)	[2]
Balance at End of Period	657,000,000		356,500,000		657,000,000		356,500,000
Hybrids [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period			5,000,000				8,800,000
Total Gains (Losses) Included in Earnings
Total Gains (Losses) Included in AOCI							(100,000)
Purchases
Sales
Settlements
Net transfer In (Out) of Level 3 (a)			(5,000,000)	[2]			(8,700,000)	[2]
Balance at End of Period
Municipals [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	34,300,000
Total Gains (Losses) Included in Earnings	0				0
Total Gains (Losses) Included in AOCI	1,300,000				600,000
Purchases	0				35,000,000
Sales	0				0
Settlements	0				0
Net transfer In (Out) of Level 3 (a)		[1]				[1]
Balance at End of Period	35,600,000				35,600,000
Equity securities available-for-sale [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period			0				0
Total Gains (Losses) Included in Earnings	0		0				0
Total Gains (Losses) Included in AOCI			0				0
Purchases			10,000,000				10,000,000
Sales			0				0
Settlements			0				0
Net transfer In (Out) of Level 3 (a)			0	[1]			0	[1]
Balance at End of Period			10,000,000				10,000,000
Held for trading [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	10,800,000				10,700,000
Total Gains (Losses) Included in Earnings					100,000
Total Gains (Losses) Included in AOCI
Purchases	0				0
Sales	0				0
Settlements	0				0
Net transfer In (Out) of Level 3 (a)	0	[1]			0	[1]
Balance at End of Period	10,800,000				10,800,000
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		41,000,000		41,000,000
Total Gains (Losses) Included in Earnings					500,000
Total Gains (Losses) Included in AOCI
Purchases
Sales
Settlements
Net transfer In (Out) of Level 3 (a)		[1]		[2]		[1]		[2]
Balance at End of Period	$ 41,500,000		$ 41,000,000		$ 41,500,000		$ 41,000,000

[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.