Carrying Amount and Fair Value of Financial Instruments (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Assets
Fixed Maturity Securities	$ 44,570.4	$ 45,064.9
Mortgage Loans	1,856.5	1,856.6
Policy Loans	3,248.4	3,306.6
Other Long-term Investments
Derivative Assets	39.3	28.0
Equity Securities	1.4	12.5
Policyholders' Funds
Long-term Debt	2,580.8	2,628.7
Payables for Collateral on Investments
Federal Home Loan Bank (FHLB) Funding Agreements	350.0
Other Liabilities
Derivative Liabilities	130.5	142.8
Unfunded Commitment to Fund Investment Partnerships	8.1
Carrying Amount
Assets
Fixed Maturity Securities	44,570.4	45,064.9
Mortgage Loans	1,856.5	1,856.6
Policy Loans	3,248.4	3,306.6
Other Long-term Investments
Derivative Assets	39.3	28.0
Equity Securities	1.4	12.5
Miscellaneous Long-Term Investments	483.3	438.7
Policyholders' Funds
Deferred Annuity Products	615.4	621.4
Supplementary Contracts without Life Contingencies	625.0	600.4
Short-term Debt	151.9	151.9
Long-term Debt	2,580.8	2,628.7
Payables for Collateral on Investments
Federal Home Loan Bank (FHLB) Funding Agreements	350.0	0.0
Other Liabilities
Derivative Liabilities	74.7	92.9
Embedded Derivative in Modified Coinsurance Arrangement	55.8	49.9
Unfunded Commitment to Fund Investment Partnerships	8.1	12.8
Fair Value
Assets
Fixed Maturity Securities	44,570.4	45,064.9
Mortgage Loans	1,995.3	2,024.2
Policy Loans	3,346.0	3,407.6
Other Long-term Investments
Derivative Assets	39.3	28.0
Equity Securities	1.4	12.5
Miscellaneous Long-Term Investments	483.3	438.7
Policyholders' Funds
Deferred Annuity Products	615.4	621.4
Supplementary Contracts without Life Contingencies	625.0	600.4
Short-term Debt	154.9	158.9
Long-term Debt	2,803.6	2,912.6
Payables for Collateral on Investments
Federal Home Loan Bank (FHLB) Funding Agreements	350.0	0.0
Other Liabilities
Derivative Liabilities	74.7	92.9
Embedded Derivative in Modified Coinsurance Arrangement	55.8	49.9
Unfunded Commitment to Fund Investment Partnerships	$ 8.1	$ 12.8