Schedule 2 - Parent Company Financial Statements - Statements of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash Flows from Operating Activities:
Net Income	$ 97.5	$ 122.3	$ 126.1	$ 64.0	$ 124.7	$ 129.0	$ 122.1	$ 155.3	$ 409.9	$ 531.1	$ 190.1
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities:
Decrease (Increase) in Value of Equity and Convertible Securities at Fair Value	(73.1)	(45.2)	(71.6)	117.8	(39.2)	(9.8)	(25.5)	(64.4)	(72.1)	(138.9)	64.3
Net Realized Investment (Gains) Losses	0.1	$ (10.0)	$ (11.7)	(16.5)	(2.8)	(1.7)	$ (21.3)	(16.1)	(38.1)	(41.9)	(26.4)
Loss from Early Extinguishment of Debt						$ 5.8			0.0	5.8	0.0
Other Assets and Liabilities									(27.8)	5.4	94.9
Net Cash Provided by Operating Activities									448.0	534.3	539.2
Cash Flows from Investing Activities:
Proceeds from Sales, Calls and Maturities of Fixed Maturities									972.4	1,229.1	2,643.3
Fixed Maturities									(1,293.3)	(1,284.9)	(2,413.2)
Equity Securities									(319.1)	(307.0)	(478.5)
Equity Securities									434.4	217.3	351.9
Net Sales (Purchases) of Short-term Investments									(390.8)	(176.0)	52.7
Acquisition of Business									0.0	0.0	(560.6)
Net Cash Provided by (Used in) Investing Activities									(757.0)	(633.4)	(497.6)
Cash Flows from Financing Activities:
Net Proceeds from Issuance of Long-term Debt									395.6	49.9	249.4
Dividends and Dividend Equivalents Paid									(78.9)	(67.8)	(56.4)
Proceeds from Shares Issued under Employee Stock Purchase Plan									4.4	1.6	0.0
Proceeds from Stock Options Exercised									5.0	2.4	0.9
Net Cash Provided by (Used in) Financing Activities									378.3	160.8	(12.2)
Cash, Beginning Balance				136.8					136.8
Cash, Ending Balance	206.1				136.8				206.1	136.8
Common Stock Repurchases									110.4	0.0	0.0
Parent Company
Cash Flows from Operating Activities:
Net Income									409.9	531.1	190.1
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities:
Equity in Net Income of Subsidiaries									(475.4)	(556.0)	(238.6)
Cash Dividends from Subsidiaries									216.2	239.0	130.4
Contribution to Defined Benefit Pension Plan									0.0	(55.3)	(5.0)
Decrease (Increase) in Value of Equity and Convertible Securities at Fair Value									(4.3)	(1.6)	(1.4)
Net Realized Investment (Gains) Losses									(0.1)	(0.3)	0.7
Loss from Early Extinguishment of Debt									0.0	5.8	0.0
Other Assets and Liabilities									52.2	9.8	29.6
Net Cash Provided by Operating Activities									262.6	172.5	105.8
Cash Flows from Investing Activities:
Capital Contributed to Subsidiaries									(62.0)	(83.0)	(20.0)
Capital Distributed from Subsidiaries									0.0	85.0	176.0
Proceeds from Sales, Calls and Maturities of Fixed Maturities									2.0	12.7	(0.2)
Equity Securities									(21.0)	(48.9)	(2.3)
Equity Securities									2.2	15.3	67.5
Net Sales (Purchases) of Short-term Investments									(415.7)	(23.3)	253.4
Acquisition of Business									0.0	0.0	(564.6)
Net Cash Provided by (Used in) Investing Activities									(494.5)	(42.2)	(90.2)
Cash Flows from Financing Activities:
Net Proceeds from Issuance of Long-term Debt									395.6	49.9	249.4
Repayment of Long-term Debt									0.0	(185.0)	(215.0)
Proceeds from Issuance or Sale of Equity									0.0	127.5	0.0
Dividends and Dividend Equivalents Paid									(78.9)	(67.8)	(56.4)
Proceeds from Shares Issued under Employee Stock Purchase Plan									4.4	1.6	0.0
Proceeds from Stock Options Exercised									5.4	2.4	0.9
Net Cash Provided by (Used in) Financing Activities									216.1	(71.4)	(21.1)
Cash, Period Increase (Decrease), Total									(15.8)	58.9	(5.5)
Cash, Beginning Balance				$ 61.8				$ 2.9	61.8	2.9	8.4
Cash, Ending Balance	$ 46.0				$ 61.8				46.0	$ 61.8	$ 2.9
Common Stock Repurchases									(110.4)
Settlement Benefits									$ 64.1