Carrying Amount and Fair Value of Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Assets
Fixed Maturity Securities	$ 45,457.8	$ 44,217.3
Mortgage Loans	2,213.2	2,038.9
Policy Loans	3,571.1	3,463.2
Other Long-term Investments
Derivative Assets	19.5	32.7
Equity Securities	11.7	1.2
Policyholders' Funds
Long-term Debt	2,738.4	2,999.4
Payables for Collateral on Investments
Advances from FHLB	350.0	350.0
Other Liabilities
Derivative Liabilities	68.1	99.5
Carrying Amount
Assets
Fixed Maturity Securities	45,457.8	44,217.3
Mortgage Loans	2,213.2	2,038.9
Policy Loans	3,571.1	3,463.2
Other Long-term Investments
Derivative Assets	19.5	32.7
Equity Securities	11.7	1.2
Miscellaneous Long-term Investments	569.5	541.9
Policyholders' Funds
Deferred Annuity Products	581.2	597.4
Supplementary Contracts without Life Contingencies	603.3	608.8
Long-term Debt	2,738.4	2,999.4
Payables for Collateral on Investments
Advances from FHLB	350.0	350.0
Other Liabilities
Derivative Liabilities	52.2	52.8
Embedded Derivative in Modified Coinsurance Arrangement	15.9	46.7
Unfunded commitment to fund Investment Partnerships	3.7	5.0
Fair Value
Assets
Fixed Maturity Securities	45,457.8	44,217.3
Mortgage Loans	2,306.2	2,122.2
Policy Loans	3,677.5	3,564.2
Other Long-term Investments
Derivative Assets	19.5	32.7
Equity Securities	11.7	1.2
Miscellaneous Long-term Investments	569.5	541.9
Policyholders' Funds
Deferred Annuity Products	581.2	597.4
Supplementary Contracts without Life Contingencies	603.3	608.8
Long-term Debt	3,048.7	3,175.8
Payables for Collateral on Investments
Advances from FHLB	350.0	350.0
Other Liabilities
Derivative Liabilities	52.2	52.8
Embedded Derivative in Modified Coinsurance Arrangement	15.9	46.7
Unfunded commitment to fund Investment Partnerships	$ 3.7	$ 5.0