Carrying Amount and Fair Value of Financial Instruments (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Assets
Mortgage Loans	$ 1,712.7	[1]	$ 1,612.3
Policy Loans	3,133.8		3,051.4
Other Miscellaneous Long Term Investments [Abstract]
Derivatives	81.6		137.7
Equity Securities	14.6		11.2
Liabilities
Long-term Debt	2,755.4		2,570.2
Other Liabilities
Derivatives	254.4		309.4
Unfunded commitment to fund Investment Partnerships	83.7
Carrying Amount
Assets
Fair value of fixed maturity securities	44,973.0		42,486.7
Mortgage Loans	1,712.7		1,612.3
Policy Loans	3,133.8		3,051.4
Other Miscellaneous Long Term Investments [Abstract]
Derivatives	81.6	[2]	137.7
Equity Securities	14.6		11.2
Miscellaneous Long-Term Investments	455.1		436.4
Liabilities
Long-term Debt	2,755.4		2,570.2
Policyholders' Funds
Deferred Annuity Products	640.1		641.1
Supplementary Contracts without Life Contingencies	535.5		502.6
Other Liabilities
Derivatives	170.5		173.7
Embedded Derivative in Modified Coinsurance Arrangement	83.9		135.7
Unfunded commitment to fund Investment Partnerships	83.7		160.6
Fair Value
Assets
Fair value of fixed maturity securities	44,973.0		42,486.7
Mortgage Loans	1,937.1		1,789.8
Policy Loans	3,215.3		3,124.4
Other Miscellaneous Long Term Investments [Abstract]
Derivatives	81.6		137.7
Equity Securities	14.6		11.2
Miscellaneous Long-Term Investments	455.1		436.4
Liabilities
Long-term Debt	2,968.8		2,540.2
Policyholders' Funds
Deferred Annuity Products	640.1		641.1
Supplementary Contracts without Life Contingencies	535.5		502.6
Other Liabilities
Derivatives	170.5		173.7
Embedded Derivative in Modified Coinsurance Arrangement	83.9		135.7
Unfunded commitment to fund Investment Partnerships	$ 83.7		$ 160.6

[1] 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 accretion of discounts.

[2] Derivatives are carried at fair value.