Schedule 2 - Parent Company Financial Statements - Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
ASSETS
Investments in Subsidiaries	$ 161.0	$ 175.9
Fixed Maturities at Fair Value (Amortized Cost: 2017 – $0.3; 2016 – $46.3)	5,382.7	5,124.9
Fair Value Option Investments	77.5	111.4
Short-term Investments	235.5	273.7
Cash	45.7	115.7	$ 161.7	$ 76.1
Other Receivables	194.3	198.6
Current Income Tax Assets	6.1	15.5
Other Assets	270.9	255.9
Total Assets	8,376.2	8,210.5
LIABILITIES AND SHAREHOLDERS’ EQUITY
Deferred Income Tax Liabilities	14.8	0.0
Accrued Expenses and Other Liabilities	453.7	453.2
Total Liabilities	6,260.6	6,235.3
Shareholders’ Equity:
Additional Paid-in Capital	673.1	660.3
Retained Earnings	1,243.0	1,172.8
Accumulated Other Comprehensive Income	194.4	137.0
Stockholders' Equity Attributable to Parent	2,115.6	1,975.2
Total Liabilities and Shareholders’ Equity	8,376.2	8,210.5
Parent Company [Member]
ASSETS
Investments in Subsidiaries	2,646.0	2,575.5
Fixed Maturities at Fair Value (Amortized Cost: 2017 – $0.3; 2016 – $46.3)	0.3	46.3
Equity Securities at Fair Value (Cost: 2017 – $6.1; 2016 – $5.2)	6.7	5.3
Fair Value Option Investments	77.5	111.4
Short-term Investments	104.4	120.0
Cash	8.4	15.7	$ 15.9	$ 17.9
Other Receivables	3.8	3.4
Current Income Tax Assets	0.0	2.4
Other Assets	5.8	7.5
Total Assets	2,852.9	2,887.5
LIABILITIES AND SHAREHOLDERS’ EQUITY
Current Income Tax Liability	14.0	0.0
Deferred Income Tax Liabilities	16.9	34.0
Liabilities for Benefit Plans	104.9	118.7
Accrued Expenses and Other Liabilities	9.2	8.0
Total Liabilities	737.3	912.3
Shareholders’ Equity:
Common Stock	5.1	5.1
Additional Paid-in Capital	673.1	660.3
Retained Earnings	1,243.0	1,172.8
Accumulated Other Comprehensive Income	194.4	137.0
Stockholders' Equity Attributable to Parent	2,115.6	1,975.2
Total Liabilities and Shareholders’ Equity	2,852.9	2,887.5
Parent Company [Member] | Senior Notes, 6% Due May 15, 2017 [Member]
LIABILITIES AND SHAREHOLDERS’ EQUITY
Notes Payable	0.0	359.8
Parent Company [Member] | Senior Notes, 4.35 Percent Due February 15, 2025 [Member]
LIABILITIES AND SHAREHOLDERS’ EQUITY
Notes Payable	448.1	247.7
Parent Company [Member] | Subordinated Debenture, 7.375 Percent Due 2054 [Member]
LIABILITIES AND SHAREHOLDERS’ EQUITY
Notes Payable	$ 144.2	$ 144.1