Fair Value of Financial Instruments - Changes in Fair Value of Financial Instruments (Detail) (USD $)	3 Months Ended				12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Sep. 30, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	$ 0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	1,066,000,000		523,500,000		523,500,000
Total Gains (Losses) Included in Earnings	500,000		600,000
Total Gains (Losses) Included in AOCI	11,100,000		(6,700,000)
Purchases	92,200,000		192,600,000
Settlements	(3,100,000)		(500,000)
Net transfer In (Out) of Level 3 (a)	(5,900,000)	[1]
Balance at End of Period	1,160,800,000		709,500,000
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	2,059,400,000		1,875,200,000		1,875,200,000
Total Gains (Losses) Included in Earnings	230,800,000		146,800,000
Total Gains (Losses) Included in AOCI	0
Purchases	346,900,000		150,600,000
Sales	0		0
Settlements	(500,000)		0
Net transfer In (Out) of Level 3	0		0
Balance at End of Period	2,636,600,000		2,172,600,000
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	1,908,100,000		1,544,400,000		1,544,400,000
Total Gains (Losses) Included in Earnings	232,100,000		100,300,000
Total Gains (Losses) Included in AOCI			0
Purchases			0
Sales			0
Settlements			0
Net transfer In (Out) of Level 3			0
Balance at End of Period	2,140,200,000		1,644,700,000
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	151,300,000
Total Gains (Losses) Included in Earnings	(1,300,000)		(700,000)
Purchases	346,900,000		150,600,000
Settlements	(500,000)
Balance at End of Period	496,400,000		149,900,000
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			330,800,000		330,800,000
Total Gains (Losses) Included in Earnings			47,200,000
Total Gains (Losses) Included in AOCI			0
Purchases			0
Sales			0
Settlements			0
Net transfer In (Out) of Level 3			0
Balance at End of Period			378,000,000
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	37,000,000		5,000,000		5,000,000
Total Gains (Losses) Included in Earnings	0
Total Gains (Losses) Included in AOCI	0		(200,000)
Purchases	3,500,000		5,000,000
Settlements	0
Net transfer In (Out) of Level 3 (a)	(5,900,000)	[1]			(5,900,000)	[1]
Balance at End of Period	34,600,000		9,800,000		37,000,000
Commercial Mortgage Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	83,100,000		5,700,000		5,700,000
Total Gains (Losses) Included in AOCI	800,000		300,000
Purchases	36,800,000
Settlements	(300,000)
Balance at End of Period	120,400,000		6,000,000
Corporate Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	850,000,000		461,100,000		461,100,000
Total Gains (Losses) Included in Earnings	(100,000)
Total Gains (Losses) Included in AOCI	8,400,000		(6,100,000)
Purchases	51,900,000		152,600,000
Sales	0
Settlements	(2,600,000)		(500,000)
Balance at End of Period	907,600,000		607,100,000
Hybrids [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total Gains (Losses) Included in AOCI			(700,000)
Purchases			35,000,000
Balance at End of Period			34,300,000
Municipals [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	37,200,000
Total Gains (Losses) Included in Earnings	0
Total Gains (Losses) Included in AOCI	1,800,000
Purchases	0
Sales	0
Settlements	(200,000)
Net transfer In (Out) of Level 3 (a)	0
Balance at End of Period	38,800,000
Equity securities available-for-sale [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	6,000,000		10,700,000		10,700,000
Total Gains (Losses) Included in Earnings			100,000
Total Gains (Losses) Included in AOCI	100,000		0
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	6,100,000		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
Nomura Fund Linked Note [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	11,200,000
Total Gains (Losses) Included in AOCI
Purchases
Sales
Settlements
Net transfer In (Out) of Level 3 (a)		[1]
Balance at End of Period	11,800,000
Embedded Derivative Financial Instruments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total Gains (Losses) Included in Earnings	600,000
Contingent Purchase Price Reduction Receivable [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period			41,000,000		41,000,000
Total Gains (Losses) Included in Earnings			500,000
Balance at End of Period	$ 41,500,000		$ 41,500,000		$ 41,500,000

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