Fair Value of Financial Instruments - Changes in Fair Value of Financial Instruments (Detail) (USD $)	3 Months Ended				9 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Document Period End Date					Jun. 30, 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	755,600,000		419,000,000		523,500,000		206,000,000
Total Gains (Losses) Included in Earnings	1,500,000				2,100,000		(300,000)
Total Gains (Losses) Included in AOCI	15,900,000		(10,800,000)		20,500,000		(9,600,000)
Purchases	191,000,000		106,300,000		425,300,000		394,700,000
Sales	(1,000,000)				(1,000,000)		(9,600,000)
Settlements	(13,500,000)		(11,800,000)		(15,900,000)		(25,500,000)
Net transfer In (Out) of Level 3 (a)	(18,900,000)	[1],[2]		[1]	(23,900,000)	[1],[2]	(53,000,000)	[1]
Balance at End of Period	930,600,000		502,700,000		930,600,000		502,700,000
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	2,234,500,000		1,842,300,000		1,875,200,000		1,782,800,000
Total Gains (Losses) Included in Earnings	112,900,000		(106,300,000)		340,900,000		(46,800,000)
Total Gains (Losses) Included in AOCI	0				0
Purchases	0		0		150,600,000
Sales	0		0		0
Settlements	(328,000,000)		(2,800,000)		(347,300,000)		(2,800,000)
Net transfer In (Out) of Level 3	0		0		0	[2]		[1]
Balance at End of Period	2,019,400,000		1,733,200,000		2,019,400,000		1,733,200,000
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	1,718,700,000		1,639,600,000		1,544,400,000		1,550,800,000
Total Gains (Losses) Included in Earnings	145,800,000		(53,700,000)		320,100,000		35,100,000
Total Gains (Losses) Included in AOCI			0
Purchases			0
Sales			0
Settlements			0
Net transfer In (Out) of Level 3		[2]	0			[2]		[1]
Balance at End of Period	1,864,500,000		1,585,900,000		1,864,500,000		1,585,900,000
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	151,000,000				0
Total Gains (Losses) Included in Earnings	5,100,000				8,100,000
Total Gains (Losses) Included in AOCI					0
Purchases					150,600,000
Sales					0
Settlements	(1,200,000)				(3,800,000)
Net transfer In (Out) of Level 3		[2]			0	[2]
Balance at End of Period	154,900,000				154,900,000
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	364,800,000		202,700,000		330,800,000		232,000,000
Total Gains (Losses) Included in Earnings	(38,000,000)		(52,600,000)		12,700,000		(81,900,000)
Total Gains (Losses) Included in AOCI			0
Purchases			0
Sales			0
Settlements	(326,800,000)		(2,800,000)		(343,500,000)		(2,800,000)
Net transfer In (Out) of Level 3		[2]	0			[2]		[1]
Balance at End of Period			147,300,000				147,300,000
Available-for-sale embedded derivatives [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total Gains (Losses) Included in AOCI			0
Asset-backed securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	10,700,000		5,300,000		5,000,000		15,900,000
Total Gains (Losses) Included in Earnings	0				0
Total Gains (Losses) Included in AOCI	0		(100,000)		(300,000)		(200,000)
Purchases	0				5,000,000
Sales
Settlements	0		(100,000)		0		(100,000)
Net transfer In (Out) of Level 3 (a)	(4,800,000)	[2]		[1]	(3,800,000)	[2]	(10,500,000)	[1]
Balance at End of Period	5,900,000		5,100,000		5,900,000		5,100,000
Commercial mortgage-backed securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period			6,200,000		5,700,000		5,000,000
Total Gains (Losses) Included in Earnings
Total Gains (Losses) Included in AOCI	100,000		(300,000)		400,000		(100,000)
Purchases	83,800,000				83,800,000		1,000,000
Sales
Settlements	0				0
Net transfer In (Out) of Level 3 (a)		[2]		[1]	(6,000,000)	[2]		[1]
Balance at End of Period	83,900,000		5,900,000		83,900,000		5,900,000
Corporates [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	657,000,000		356,500,000		461,100,000		135,300,000
Total Gains (Losses) Included in Earnings	0				0		(300,000)
Total Gains (Losses) Included in AOCI	14,400,000		(12,000,000)		18,400,000		(10,800,000)
Purchases	88,900,000		106,200,000		283,200,000		383,600,000
Sales	(1,000,000)				(1,000,000)		(9,600,000)
Settlements	0		(11,700,000)		(2,400,000)		(25,400,000)
Net transfer In (Out) of Level 3 (a)	(14,100,000)	[2]		[1]	(14,100,000)	[2]	(33,800,000)	[1]
Balance at End of Period	745,200,000		439,000,000		745,200,000		439,000,000
Hybrids [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period							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)							(8,700,000)	[1]
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	35,600,000
Total Gains (Losses) Included in Earnings	0				0
Total Gains (Losses) Included in AOCI	900,000				1,500,000
Purchases	0				35,000,000
Sales	0				0
Settlements	0				0
Net transfer In (Out) of Level 3 (a)	0					[2]
Balance at End of Period	36,500,000				36,500,000
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		10,000,000		0		0
Total Gains (Losses) Included in Earnings	1,200,000		0		0		0
Total Gains (Losses) Included in AOCI	500,000		1,600,000		500,000		1,600,000
Purchases	5,500,000		100,000		5,500,000		10,100,000
Sales	0		0		0		0
Settlements	0		0		0		0
Net transfer In (Out) of Level 3 (a)	0	[2]	0	[2]	0	[2]	0	[2]
Balance at End of Period	6,000,000		11,700,000		6,000,000		11,700,000
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	0				1,300,000
Total Gains (Losses) Included in AOCI
Purchases	1,500,000				1,500,000
Sales	0				0
Settlements	(13,500,000)				(13,500,000)
Net transfer In (Out) of Level 3 (a)	0	[2]			0	[2]
Balance at End of Period	0				0
Nomura Fund Linked Note [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 AOCI	0				0
Purchases	11,300,000				11,300,000
Sales	0				0
Settlements	0				0
Net transfer In (Out) of Level 3 (a)	0	[2]			0	[2]
Balance at End of Period	11,600,000				11,600,000
Embedded Derivative Financial Instruments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total Gains (Losses) Included in Earnings	300,000				300,000
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)		[2]		[1]		[2]		[1]
Balance at End of Period	$ 41,500,000		$ 41,000,000		$ 41,500,000		$ 41,000,000

[1] (a)The net transfers in and out of Level 3 during the nine months ended June 30, 2013 were exclusively to or from Level 2.

[2] 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.