Carrying Amount and Fair Value of Financial Instruments (Detail) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Assets
Fixed Maturity Securities	$ 44,623.2	$ 44,217.3
Mortgage Loans	2,068.4	2,038.9
Policy Loans	3,447.4	3,463.2
Other Long-term Investments
Derivative Assets	29.3	32.7
Equity Securities	1.2	1.2
Policyholders' Funds
Long-term Debt	2,984.6	2,999.4
Payables for Collateral on Investments
Federal Home Loan Bank (FHLB) Funding Agreements	350.0	350.0
Other Liabilities
Derivative Liabilities	91.1	99.5
Unfunded Commitments to Investment Partnerships	4.4
Fair Value
Assets
Fixed Maturity Securities	44,623.2	44,217.3
Mortgage Loans	2,162.4	2,122.2
Policy Loans	3,548.2	3,564.2
Other Long-term Investments
Derivative Assets	29.3	32.7
Equity Securities	1.2	1.2
Miscellaneous Long-Term Investments	559.3	541.9
Policyholders' Funds
Deferred Annuity Products	594.5	597.4
Supplementary Contracts without Life Contingencies	610.9	608.8
Long-term Debt	3,222.7	3,175.8
Payables for Collateral on Investments
Federal Home Loan Bank (FHLB) Funding Agreements	350.0	350.0
Other Liabilities
Derivative Liabilities	53.0	52.8
Embedded Derivative in Modified Coinsurance Arrangement	38.1	46.7
Unfunded Commitments to Investment Partnerships	4.7	5.0
Carrying Amount
Assets
Fixed Maturity Securities	44,623.2	44,217.3
Mortgage Loans	2,068.4	2,038.9
Policy Loans	3,447.4	3,463.2
Other Long-term Investments
Derivative Assets	29.3	32.7
Equity Securities	1.2	1.2
Miscellaneous Long-Term Investments	559.3	541.9
Policyholders' Funds
Deferred Annuity Products	594.5	597.4
Supplementary Contracts without Life Contingencies	610.9	608.8
Long-term Debt	2,984.6	2,999.4
Payables for Collateral on Investments
Federal Home Loan Bank (FHLB) Funding Agreements	350.0	350.0
Other Liabilities
Derivative Liabilities	53.0	52.8
Embedded Derivative in Modified Coinsurance Arrangement	38.1	46.7
Unfunded Commitments to Investment Partnerships	$ 4.7	$ 5.0

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