Fair Values of Financial Instruments Fair Value, by Balance Sheet Groupings (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Mortgage Loans, Fair Value	$ 2,235.1	$ 2,306.2
Mortgage Loans, Carrying Value	2,200.8	2,213.2
Policy Loans, Fair Value	3,666.0	3,677.5
Policy Loans, Carrying Value	3,573.3	3,571.1
Other Long-term Investments
Miscellaneous Long-Term Investments, Fair Value	153.2	162.3
Miscellaneous Long-Term Investments, Carrying Value	153.2	162.3
Total Financial Instruments Assets Not Carried at Fair Value	6,054.3	6,146.0
Financial Instruments, Financial Assets Fair Value Disclosure, Carrying Value	5,927.3	5,946.6
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term Debt, Fair Value	2,961.6	3,048.7
Long-term Debt, Carrying Value	2,721.9	2,738.4
Collateralized Financings [Abstract]
Federal Home Loan Bank (FHLB) Funding Agreements, Fair Value	350.0	350.0
Federal Home Loan Bank (FHLB) Funding Agreements, Carrying Value	350.0	350.0
Other Liabilities, Unfunded Commitments	3.7	3.7
Total Financial Liabilities, Fair Value	3,315.3	3,402.4
Total Financial Liabilities, Carrying Value	3,075.6	3,092.1
Fair Value, Inputs, Level 1
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Mortgage Loans, Fair Value	0.0	0.0
Policy Loans, Fair Value	0.0	0.0
Other Long-term Investments
Miscellaneous Long-Term Investments, Fair Value	0.0	0.0
Total Financial Instruments Assets Not Carried at Fair Value	0.0	0.0
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term Debt, Fair Value	2,155.7	1,171.8
Collateralized Financings [Abstract]
Federal Home Loan Bank (FHLB) Funding Agreements, Fair Value	0.0	0.0
Other Liabilities, Unfunded Commitments	0.0	0.0
Total Financial Liabilities, Fair Value	2,155.7	1,171.8
Fair Value, Inputs, Level 2
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Mortgage Loans, Fair Value	2,235.1	2,306.2
Policy Loans, Fair Value	0.0	0.0
Other Long-term Investments
Miscellaneous Long-Term Investments, Fair Value	34.1	34.1
Total Financial Instruments Assets Not Carried at Fair Value	2,269.2	2,340.3
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term Debt, Fair Value	805.9	1,876.9
Collateralized Financings [Abstract]
Federal Home Loan Bank (FHLB) Funding Agreements, Fair Value	350.0	350.0
Other Liabilities, Unfunded Commitments	3.7	3.7
Total Financial Liabilities, Fair Value	1,159.6	2,230.6
Fair Value, Inputs, Level 3
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Mortgage Loans, Fair Value	0.0	0.0
Policy Loans, Fair Value	3,666.0	3,677.5
Other Long-term Investments
Miscellaneous Long-Term Investments, Fair Value	119.1	128.2
Total Financial Instruments Assets Not Carried at Fair Value	3,785.1	3,805.7
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term Debt, Fair Value	0.0	0.0
Collateralized Financings [Abstract]
Federal Home Loan Bank (FHLB) Funding Agreements, Fair Value	0.0	0.0
Other Liabilities, Unfunded Commitments	0.0	0.0
Total Financial Liabilities, Fair Value	$ 0.0	$ 0.0