Fair Value of Financial Instruments - Changes in Fair Value of Financial Instruments (Detail) (USD $)	3 Months Ended				6 Months Ended
	Mar. 31, 2015		Mar. 31, 2014		Mar. 31, 2015		Mar. 31, 2014		Dec. 31, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	$ 0				$ 0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	1,160,800,000		709,500,000		1,066,000,000		523,500,000
Total Gains (Losses) Included in Earnings	8,400,000				8,900,000		600,000
Total Gains (Losses) Included in AOCI	17,300,000		11,300,000		28,400,000		4,600,000
Purchases	94,600,000		41,700,000		186,800,000		234,300,000
Settlements	(30,600,000)		(1,900,000)		(33,700,000)		(2,400,000)
Net transfer In (Out) of Level 3 (a)	(27,600,000)	[1]	(5,000,000)	[2]	(33,500,000)	[3]	(5,000,000)	[4]
Balance at End of Period	1,222,900,000		755,600,000		1,222,900,000		755,600,000
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	2,636,600,000		2,172,600,000		2,059,400,000		1,875,200,000
Total Gains (Losses) Included in Earnings	84,900,000		81,200,000		315,700,000		228,000,000
Total Gains (Losses) Included in AOCI	0				0
Purchases	90,100,000		0		437,000,000		150,600,000
Sales	0		0		0		0
Settlements	(9,700,000)		(19,300,000)		(10,200,000)		(19,300,000)
Net transfer In (Out) of Level 3	0	[1]	0	[2]	0	[3]	0	[4]
Balance at End of Period	2,801,900,000		2,234,500,000		2,801,900,000		2,234,500,000
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	2,140,200,000		1,644,700,000		1,908,100,000		1,544,400,000
Total Gains (Losses) Included in Earnings	77,200,000		74,000,000		309,300,000		174,300,000
Total Gains (Losses) Included in AOCI			0				0
Purchases			0				0
Sales			0				0
Settlements			0				0
Net transfer In (Out) of Level 3			0	[2]			0	[4]
Balance at End of Period	2,217,400,000		1,718,700,000		2,217,400,000		1,718,700,000
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	496,400,000		149,900,000		151,300,000
Total Gains (Losses) Included in Earnings	7,700,000		3,700,000		6,400,000		3,000,000
Purchases	90,100,000				437,000,000		150,600,000
Settlements	(9,700,000)		(2,600,000)		(10,200,000)		(2,600,000)
Balance at End of Period	584,500,000		151,000,000		584,500,000		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				330,800,000
Total Gains (Losses) Included in Earnings			3,500,000				50,700,000
Total Gains (Losses) Included in AOCI			0				0
Purchases			0				0
Sales			0				0
Settlements			(16,700,000)				(16,700,000)
Net transfer In (Out) of Level 3			0	[2]			0	[4]
Balance at End of Period			364,800,000				364,800,000
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				0
Asset-backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	34,600,000		9,800,000		37,000,000		5,000,000
Total Gains (Losses) Included in Earnings	100,000				100,000
Total Gains (Losses) Included in AOCI	0		(100,000)		0		(300,000)
Purchases	3,200,000				6,700,000		5,000,000
Settlements	(100,000)				(100,000)
Net transfer In (Out) of Level 3 (a)	(3,500,000)	[1]	1,000,000	[2]	(9,400,000)	[3]	1,000,000	[4]
Balance at End of Period	34,300,000		10,700,000		34,300,000		10,700,000
Commercial Mortgage Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	120,400,000		6,000,000		83,100,000		5,700,000
Total Gains (Losses) Included in AOCI	1,500,000				2,300,000		300,000
Purchases	20,900,000				57,700,000
Settlements	(300,000)				(600,000)
Net transfer In (Out) of Level 3 (a)			(6,000,000)	[2]			(6,000,000)	[4]
Balance at End of Period	142,500,000				142,500,000
Corporate Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	907,600,000		607,100,000		850,000,000		461,100,000
Total Gains (Losses) Included in Earnings	2,400,000				2,300,000
Total Gains (Losses) Included in AOCI	14,900,000		10,100,000		23,300,000		4,000,000
Purchases	70,500,000		41,700,000		122,400,000		194,300,000
Sales	0				0
Settlements	(30,200,000)		(1,900,000)		(32,800,000)		(2,400,000)
Net transfer In (Out) of Level 3 (a)	(24,100,000)	[1]			(24,100,000)	[3]
Balance at End of Period	941,100,000		657,000,000		941,100,000		657,000,000
Hybrids [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 AOCI			1,300,000				600,000
Purchases							35,000,000
Balance at End of Period			35,600,000				35,600,000
Municipals [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	38,800,000				37,200,000
Total Gains (Losses) Included in Earnings	0				0
Total Gains (Losses) Included in AOCI	1,000,000				2,800,000
Purchases	0				0
Sales	0				0
Settlements	0				(200,000)
Net transfer In (Out) of Level 3 (a)	0	[1]			0	[3]
Balance at End of Period	39,800,000				39,800,000
Equity securities available-for-sale [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	6,100,000		10,800,000		6,000,000		10,700,000
Total Gains (Losses) Included in Earnings			0				100,000
Total Gains (Losses) Included in AOCI	(100,000)		0		0		0
Purchases	0		0		0		0
Sales	0		0		0		0
Settlements	0		0		0		0
Net transfer In (Out) of Level 3 (a)	0	[1]	0	[2]	0	[3]	0	[4]
Balance at End of Period	6,000,000		10,800,000		6,000,000		10,800,000
Held for trading [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total Gains (Losses) Included in Earnings	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	11,800,000				11,200,000
Total Gains (Losses) Included in AOCI	0				0
Purchases	0				0
Sales	0				0
Settlements	0				0
Net transfer In (Out) of Level 3 (a)	0	[1]			0	[3]
Balance at End of Period	12,200,000				12,200,000
Embedded Derivative Financial Instruments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total Gains (Losses) Included in Earnings	400,000				1,000,000
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				41,500,000		41,000,000		41,500,000
Total Gains (Losses) Included in Earnings	5,500,000				5,500,000		500,000
Balance at End of Period	$ 47,000,000		$ 41,500,000		$ 47,000,000		$ 41,500,000		$ 41,500,000

[1] (a) The net transfers out of Level 3 during the three months ended March 31, 2015 were exclusively to Level 2.

[2] (a) The net transfers out of Level 3 during the three months ended March 31, 2014 were exclusively to Level 2. There was a $6.0 transfer to asset-backed securities from commercial mortgage-backed securities

[3] (a) The net transfers out of Level 3 during the six months ended March 31, 2015 were exclusively to Level 2.

[4] (a) The net transfers out of Level 3 during the six months ended March 31, 2015 were exclusively to Level 2. There was a $6.0 transfer to asset-backed securities from commercial mortgage-backed securities