Schedule 2 - Parent Company Financial Statements - Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
ASSETS
Investments in Subsidiaries	$ 220.4	$ 187.0
Fixed Maturities at Fair Value (Amortized Cost: 2018 – $12.6)	6,922.1	6,424.2
Equity Securities at Fair Value	907.3	684.4
Equity Securities at Modified Cost	41.9	41.5
Short-term Investments at Cost which Approximates Fair Value	470.9	286.1
Cash	136.8	75.1	$ 45.7	$ 115.7
Other Receivables	219.7	245.4
Other Assets	774.7	526.5
Operating Lease Right-of-Use Assets	75.6
Total Assets	12,989.1	11,544.9
LIABILITIES AND SHAREHOLDERS’ EQUITY
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2019 - $820.2; 2018 - $911.2)	778.4	909.0
Deferred Income Tax Liabilities	178.2	26.2
Present Value of Minimum Lease Payments	93.2
Accrued Expenses and Other Liabilities	733.1	687.3
Total Liabilities	9,016.8	8,494.8
Shareholders’ Equity:
Common Stock	6.7	6.5
Additional Paid-in Capital	1,819.2	1,666.3
Retained Earnings	1,810.3	1,355.5
Accumulated Other Comprehensive Income	336.1	21.8
Total Shareholders’ Equity	3,972.3	3,050.1
Total Liabilities and Shareholders’ Equity	12,989.1	11,544.9
Parent Company
ASSETS
Investments in Subsidiaries	4,383.7	3,736.0
Fixed Maturities at Fair Value (Amortized Cost: 2018 – $12.6)	0.0	12.6
Equity Securities at Fair Value	55.7	20.4
Short-term Investments at Cost which Approximates Fair Value	89.3	64.6
Cash	61.8	2.9	$ 8.4	$ 15.7
Other Receivables	21.9	5.7
Other Assets	21.9	14.4
Operating Lease Right-of-Use Assets	18.6	0.0
Total Assets	4,652.9	3,856.6
LIABILITIES AND SHAREHOLDERS’ EQUITY
Current Income Tax Liability	55.3	40.5
Deferred Income Tax Liabilities	32.2	20.2
Liabilities for Benefit Plans	44.3	98.2
Present Value of Minimum Lease Payments	31.3	0.0
Accrued Expenses and Other Liabilities	19.0	20.1
Total Liabilities	680.6	806.5
Shareholders’ Equity:
Common Stock	6.7	6.5
Additional Paid-in Capital	1,819.2	1,666.3
Retained Earnings	1,810.3	1,355.5
Accumulated Other Comprehensive Income	336.1	21.8
Total Shareholders’ Equity	3,972.3	3,050.1
Total Liabilities and Shareholders’ Equity	4,652.9	3,856.6
Parent Company | Term Loan due June 29, 2020 (Fair Value: 2018 – $35.0)
LIABILITIES AND SHAREHOLDERS’ EQUITY
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2019 - $820.2; 2018 - $911.2)	0.0	34.9
Parent Company | Term Loan due July 5, 2023 (Fair Value: 2019 – $50.0)
LIABILITIES AND SHAREHOLDERS’ EQUITY
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2019 - $820.2; 2018 - $911.2)	49.9	0.0
Parent Company | Senior Notes Payable, 4.35% due 2025 (Fair Value: 2019 – $478.6; 2018 – $444.2)
LIABILITIES AND SHAREHOLDERS’ EQUITY
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2019 - $820.2; 2018 - $911.2)	448.6	448.4
Parent Company | Subordinated Debentures due 2054 (Fair Value: 2018 – $151.1)
LIABILITIES AND SHAREHOLDERS’ EQUITY
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2019 - $820.2; 2018 - $911.2)	$ 0.0	$ 144.2