Carrying Amount and Fair Value of Financial Instruments (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Assets
Fixed Maturity Securities	$ 44,401.5	$ 45,064.9
Mortgage Loans	1,925.9	1,856.6
Policy Loans	3,406.0	3,306.6
Other Long-term Investments
Derivative Assets	57.1	28.0
Equity Securities	1.5	12.5
Policyholders' Funds
Long-term Debt	2,217.1	2,628.7
Payables for Collateral on Investments
Federal Home Loan Bank (FHLB) Funding Agreements	350.0
Other Liabilities
Derivative Liabilities	139.6	142.8
Unfunded Commitments to Investment Partnerships	7.2
Carrying Amount
Assets
Fixed Maturity Securities	44,401.5	45,064.9
Mortgage Loans	1,925.9	1,856.6
Policy Loans	3,406.0	3,306.6
Other Long-term Investments
Derivative Assets	57.1	28.0
Equity Securities	1.5	12.5
Miscellaneous Long-Term Investments	470.9	438.7
Policyholders' Funds
Deferred Annuity Products	614.3	621.4
Supplementary Contracts without Life Contingencies	634.3	600.4
Short-term Debt	506.1	151.9
Long-term Debt	2,217.1	2,628.7
Payables for Collateral on Investments
Federal Home Loan Bank (FHLB) Funding Agreements	350.0	0.0
Other Liabilities
Derivative Liabilities	56.5	92.9
Embedded Derivative in Modified Coinsurance Arrangement	83.1	49.9
Unfunded Commitments to Investment Partnerships	7.2	12.8
Fair Value
Assets
Fixed Maturity Securities	44,401.5	45,064.9
Mortgage Loans	2,095.5	2,024.2
Policy Loans	3,513.7	3,407.6
Other Long-term Investments
Derivative Assets	57.1	28.0
Equity Securities	1.5	12.5
Miscellaneous Long-Term Investments	470.9	438.7
Policyholders' Funds
Deferred Annuity Products	614.3	621.4
Supplementary Contracts without Life Contingencies	634.3	600.4
Short-term Debt	526.4	158.9
Long-term Debt	2,430.9	2,912.6
Payables for Collateral on Investments
Federal Home Loan Bank (FHLB) Funding Agreements	350.0	0.0
Other Liabilities
Derivative Liabilities	56.5	92.9
Embedded Derivative in Modified Coinsurance Arrangement	83.1	49.9
Unfunded Commitments to Investment Partnerships	$ 7.2	$ 12.8