Carrying Amount and Fair Value of Financial Instruments (Detail) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Assets
Fixed Maturity Securities	$ 46,315.5	$ 43,354.4
Mortgage Loans	1,970.4	1,883.6
Policy Loans	3,432.8	3,395.4
Other Long-term Investments
Derivative Assets	37.1	49.8
Equity Securities	1.2	1.4
Policyholders' Funds
Long-term Debt	3,019.8	2,449.4
Payables for Collateral on Investments
Federal Home Loan Bank (FHLB) Funding Agreements	349.5	350.0
Other Liabilities
Derivative Liabilities	129.0	137.8
Unfunded Commitments to Investment Partnerships	4.8
Carrying Amount
Assets
Fixed Maturity Securities	46,315.5	43,354.4
Mortgage Loans	1,970.4	1,883.6
Policy Loans	3,432.8	3,395.4
Other Long-term Investments
Derivative Assets	37.1	49.8
Equity Securities	1.2	1.4
Miscellaneous Long-Term Investments	540.3	474.4
Policyholders' Funds
Deferred Annuity Products	602.7	608.8
Supplementary Contracts without Life Contingencies	603.7	641.1
Short-term Debt	0.0	352.0
Long-term Debt	3,019.8	2,449.4
Payables for Collateral on Investments
Federal Home Loan Bank (FHLB) Funding Agreements	349.5	350.0
Other Liabilities
Derivative Liabilities	55.7	50.2
Embedded Derivative in Modified Coinsurance Arrangement	73.3	87.6
Unfunded Commitments to Investment Partnerships	5.4	5.0
Fair Value
Assets
Fixed Maturity Securities	46,315.5	43,354.4
Mortgage Loans	2,137.6	2,013.9
Policy Loans	3,556.7	3,498.0
Other Long-term Investments
Derivative Assets	37.1	49.8
Equity Securities	1.2	1.4
Miscellaneous Long-Term Investments	540.3	474.4
Policyholders' Funds
Deferred Annuity Products	602.7	608.8
Supplementary Contracts without Life Contingencies	603.7	641.1
Short-term Debt	0.0	366.2
Long-term Debt	3,319.8	2,645.9
Payables for Collateral on Investments
Federal Home Loan Bank (FHLB) Funding Agreements	349.5	350.0
Other Liabilities
Derivative Liabilities	55.7	50.2
Embedded Derivative in Modified Coinsurance Arrangement	73.3	87.6
Unfunded Commitments to Investment Partnerships	$ 5.4	$ 5.0

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