Carrying Amount and Fair Value of Financial Instruments (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012
Assets
Fixed Maturity Securities	$ 42,344.4		$ 44,973.0
Mortgage Loans	1,815.1	[1]	1,712.7
Policy Loans	3,276.0	[1]	3,133.8
Other Long-term Investments
Equity Securities	16.4		14.6
Policyholders' Funds
Long-term Debt	2,612.0		2,755.4
Other Liabilities
Unfunded commitment to fund Investment Partnerships	27.2
Carrying Amount
Assets
Fixed Maturity Securities	42,344.4		44,973.0
Mortgage Loans	1,815.1		1,712.7
Policy Loans	3,276.0		3,133.8
Other Long-term Investments
Derivative Assets	10.8	[2]	81.6
Equity Securities	16.4		14.6
Miscellaneous Long-Term Investments	475.2		455.1
Policyholders' Funds
Deferred Annuity Products	631.5		640.1
Supplementary Contracts without Life Contingencies	563.1		535.5
Long-term Debt	2,612.0		2,755.4
Other Liabilities
Derivatives	135.6		170.5
Embedded Derivative in Modified Coinsurance Arrangement	53.2		83.9
Unfunded commitment to fund Investment Partnerships	27.2		83.7
Fair Value
Assets
Fixed Maturity Securities	42,344.4		44,973.0
Mortgage Loans	1,980.2		1,937.1
Policy Loans	3,339.6		3,215.3
Other Long-term Investments
Derivative Assets	10.8		81.6
Equity Securities	16.4		14.6
Miscellaneous Long-Term Investments	475.2		455.1
Policyholders' Funds
Deferred Annuity Products	631.5		640.1
Supplementary Contracts without Life Contingencies	563.1		535.5
Long-term Debt	2,824.4		2,968.8
Other Liabilities
Derivatives	135.6		170.5
Embedded Derivative in Modified Coinsurance Arrangement	53.2		83.9
Unfunded commitment to fund Investment Partnerships	$ 27.2		$ 83.7

[1] (1)The amortized cost for fixed maturity securities and mortgage loans represents original cost reduced by repayments, write-downs from other-than-temporary declines in fair value, amortization of premiums, and/or accretion of discounts.

[2] (2)Derivatives are carried at fair value.