Carrying Amount and Fair Value of Financial Instruments (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Assets
Fixed Maturity Securities	$ 45,422.6	$ 44,217.3
Mortgage Loans	2,145.3	2,038.9
Policy Loans	3,567.5	3,463.2
Other Long-term Investments
Derivative Assets	20.6	32.7
Equity Securities	1.3	1.2
Policyholders' Funds
Long-term Debt	2,754.1	2,999.4
Payables for Collateral on Investments
Federal Home Loan Bank (FHLB) Funding Agreements	350.0	350.0
Other Liabilities
Derivative Liabilities	77.4	99.5
Unfunded Commitments to Investment Partnerships	4.4
Fair Value
Assets
Fixed Maturity Securities	45,422.6	44,217.3
Mortgage Loans	2,246.7	2,122.2
Policy Loans	3,674.7	3,564.2
Other Long-term Investments
Derivative Assets	20.6	32.7
Equity Securities	1.3	1.2
Miscellaneous Long-Term Investments	577.6	541.9
Policyholders' Funds
Deferred Annuity Products	589.9	597.4
Supplementary Contracts without Life Contingencies	602.2	608.8
Long-term Debt	3,054.5	3,175.8
Payables for Collateral on Investments
Federal Home Loan Bank (FHLB) Funding Agreements	350.0	350.0
Other Liabilities
Derivative Liabilities	52.2	52.8
Embedded Derivative in Modified Coinsurance Arrangement	25.2	46.7
Unfunded Commitments to Investment Partnerships	4.5	5.0
Carrying Amount
Assets
Fixed Maturity Securities	45,422.6	44,217.3
Mortgage Loans	2,145.3	2,038.9
Policy Loans	3,567.5	3,463.2
Other Long-term Investments
Derivative Assets	20.6	32.7
Equity Securities	1.3	1.2
Miscellaneous Long-Term Investments	577.6	541.9
Policyholders' Funds
Deferred Annuity Products	589.9	597.4
Supplementary Contracts without Life Contingencies	602.2	608.8
Long-term Debt	2,754.1	2,999.4
Payables for Collateral on Investments
Federal Home Loan Bank (FHLB) Funding Agreements	350.0	350.0
Other Liabilities
Derivative Liabilities	52.2	52.8
Embedded Derivative in Modified Coinsurance Arrangement	25.2	46.7
Unfunded Commitments to Investment Partnerships	$ 4.5	$ 5.0