Schedule 2 - Parent Company Financial Statements - Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Jul. 05, 2019	Dec. 31, 2018	Dec. 31, 2017
ASSETS
Investments in Subsidiaries	$ 225.3	$ 220.4
Fixed Maturities at Fair Value (Amortized Cost: 2020 – $91.8)	7,605.9	6,922.1
Equity Securities at Fair Value (Cost: 2020 - $73.4; 2019 - $53.4)	858.5	907.3
Equity Securities at Modified Cost	40.1	41.9
Cash	206.1	136.8
Other Receivables	222.4	219.7
Other Assets	575.9	557.7
Operating Lease Right-of-Use Assets	68.6	75.6
Total Assets	14,341.9	12,989.1
LIABILITIES AND SHAREHOLDERS’ EQUITY
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2020 - $1,247.8; 2019 - $820.2)	1,172.8	778.4
Deferred Income Tax Liabilities	285.7	178.2
Present Value of Minimum Lease Payments	89.6	93.2
Accrued Expenses and Other Liabilities	727.9	733.1
Liabilities	9,778.5	9,016.8
Shareholders’ Equity:
Common Stock	6.5	6.7
Additional Paid-in Capital	1,805.2	1,819.2
Retained Earnings	2,071.2	1,810.3
Accumulated Other Comprehensive Income	680.5	336.1
Total Shareholders’ Equity	4,563.4	3,972.3
Total Liabilities and Shareholders’ Equity	14,341.9	12,989.1
Amortized cost of fixed maturities	6,692.7	6,372.7
Cost of equity securities	684.1	818.8
Debt	1,247.8	820.2
Term Loan due July 5, 2023 (Fair Value: 2020 – $50.0; 2019 – $50.0) | Secured Debt
LIABILITIES AND SHAREHOLDERS’ EQUITY
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2020 - $1,247.8; 2019 - $820.2)	49.9	49.9	$ 49.9
Senior Notes Payable, 4.35% due 2025 (Fair Value: 2020 – $499.5; 2019 – $478.6) | Senior Notes
LIABILITIES AND SHAREHOLDERS’ EQUITY
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2020 - $1,247.8; 2019 - $820.2)	448.8	448.6
Senior Notes, 2.400 Percent Due September 30, 2030 | Senior Notes
LIABILITIES AND SHAREHOLDERS’ EQUITY
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2020 - $1,247.8; 2019 - $820.2)	395.8	0.0
Parent Company
ASSETS
Investments in Subsidiaries	4,896.0	4,383.7
Fixed Maturities at Fair Value (Amortized Cost: 2020 – $91.8)	99.8	0.0
Equity Securities at Fair Value (Cost: 2020 - $73.4; 2019 - $53.4)	78.8	55.7
Short-term Investments at Cost which Approximates Fair Value	508.2	89.3
Cash	46.0	61.8		$ 2.9	$ 8.4
Other Receivables	1.1	21.9
Other Assets	29.2	21.9
Operating Lease Right-of-Use Assets	13.4	18.6
Total Assets	5,672.5	4,652.9
LIABILITIES AND SHAREHOLDERS’ EQUITY
Current Income Tax Liability	41.5	55.3
Deferred Income Tax Liabilities	38.9	32.2
Liabilities for Benefit Plans	47.6	44.3
Present Value of Minimum Lease Payments	26.9	31.3
Accrued Expenses and Other Liabilities	59.7	19.0
Liabilities	1,109.1	680.6
Shareholders’ Equity:
Common Stock	6.5	6.7
Additional Paid-in Capital	1,805.2	1,819.2
Retained Earnings	2,071.2	1,810.3
Accumulated Other Comprehensive Income	680.5	336.1
Total Shareholders’ Equity	4,563.4	3,972.3
Total Liabilities and Shareholders’ Equity	5,672.5	4,652.9
Amortized cost of fixed maturities	91.8
Cost of equity securities	73.4	53.4
Parent Company | Term Loan due July 5, 2023 (Fair Value: 2020 – $50.0; 2019 – $50.0)
LIABILITIES AND SHAREHOLDERS’ EQUITY
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2020 - $1,247.8; 2019 - $820.2)	49.9	49.9
Parent Company | Term Loan due July 5, 2023 (Fair Value: 2020 – $50.0; 2019 – $50.0) | Secured Debt
Shareholders’ Equity:
Debt	50.0	50.0
Parent Company | Senior Notes Payable, 4.35% due 2025 (Fair Value: 2020 – $499.5; 2019 – $478.6)
LIABILITIES AND SHAREHOLDERS’ EQUITY
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2020 - $1,247.8; 2019 - $820.2)	448.8	448.6
Parent Company | Senior Notes Payable, 4.35% due 2025 (Fair Value: 2020 – $499.5; 2019 – $478.6) | Senior Notes
Shareholders’ Equity:
Debt	499.5	$ 478.6
Parent Company | Senior Notes, 2.400 Percent Due September 30, 2030
LIABILITIES AND SHAREHOLDERS’ EQUITY
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2020 - $1,247.8; 2019 - $820.2)	395.8
Parent Company | Senior Notes, 2.400 Percent Due September 30, 2030 | Senior Notes
Shareholders’ Equity:
Debt	$ 405.6