Schedule 2 - Parent Company Financial Statements - Statements of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Activities:
Net Income	$ 124.7	$ 129.0	$ 122.1	$ 155.3	$ 6.5	$ 92.2	$ 37.6	$ 53.8	$ 531.1	$ 190.1	$ 120.9
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities:
Equity in Earnings of Equity Method Limited Liability Investments									(1.0)	(11.0)	(24.8)
Increase in Value of Equity Securities at Fair Value	(39.2)	(9.8)	(25.5)	(64.4)	76.4	(11.0)	(0.4)	(0.7)	(138.9)	64.3	0.0
Net Realized Gains on Sales of Investments	(2.8)	(1.7)	(21.3)	(16.1)	(16.4)	$ (3.6)	$ (3.8)	(2.6)	(41.9)	(26.4)	(56.5)
Loss from Early Extinguishment of Debt	0.0	$ 5.8	$ 0.0	0.0					5.8	0.0	0.0
Other, Net									30.7	13.7	(2.1)
Net Cash Provided by Operating Activities									534.3	539.2	240.6
Investing Activities:
Sales, Paydowns and Maturities of Fixed Maturities									1,229.1	2,643.3	528.2
Purchases of Fixed Maturities									(1,284.9)	(2,413.2)	(710.1)
Purchases of Equity and Convertible Securities									(307.0)	(478.5)	(342.7)
Sales of Equity and Convertible Securities									217.3	351.9	342.0
Sales of Fair Value Option Investments									0.0	0.0	42.2
Purchases of Fair Value Option Investments									0.0	0.0	(7.0)
Acquisition of Business									0.0	(560.6)	0.0
Change in Short-term Investments									(176.0)	52.7	39.4
Net Cash Used by Investing Activities									(633.4)	(497.6)	(105.4)
Financing Activities:
Net Proceeds from Issuance of Long-term Debt									49.9	249.4	200.2
Repayment of Long-term Debt									(185.0)	(215.0)	(360.0)
Proceeds from Issuance of Common Stock, Net of Transaction Costs									127.5	0.0	0.0
Dividends and Dividend Equivalents Paid									(67.8)	(56.4)	(49.5)
Proceeds from Shares Issued under Employee Stock Purchase Plan									1.6	0.0	0.0
Cash Exercise of Stock Options									2.4	0.9	4.0
Net Cash Provided (Used) by Financing Activities									160.8	(12.2)	(205.2)
Increase (Decrease) in Cash									61.7	29.4	(70.0)
Cash, Beginning of Year				75.1				45.7	75.1	45.7	115.7
Cash, End of Year	136.8				75.1				136.8	75.1	45.7
Parent Company
Operating Activities:
Net Income									531.1	190.1	120.9
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities:
Equity in Earnings of Equity Method Limited Liability Investments									(556.0)	(238.6)	(127.6)
Cash Dividends from Subsidiaries									239.0	130.4	108.1
Cash Contribution to Defined Benefit Plan									(55.3)	(5.0)	0.0
Increase in Value of Equity Securities at Fair Value									(1.6)	(1.4)	0.0
Net Realized Gains on Sales of Investments									(0.3)	0.7	(0.6)
Loss from Early Extinguishment of Debt									5.8	0.0	0.0
Other, Net									9.8	29.6	0.7
Net Cash Provided by Operating Activities									172.5	105.8	101.5
Investing Activities:
Capital Contributed to Subsidiaries									(83.0)	(20.0)	0.0
Capital Distribution from Subsidiary									85.0	176.0	0.0
Sales, Paydowns and Maturities of Fixed Maturities									12.7	(0.2)	45.7
Purchases of Equity and Convertible Securities									(48.9)	(2.3)	0.0
Sales of Equity and Convertible Securities									15.3	67.5	0.0
Sales of Fair Value Option Investments									0.0	0.0	42.2
Purchases of Fair Value Option Investments									0.0	0.0	(7.0)
Acquisition of Business									0.0	(564.6)	0.0
Change in Short-term Investments									(23.3)	253.4	15.6
Net Cash Used by Investing Activities									(42.2)	(90.2)	96.5
Financing Activities:
Net Proceeds from Issuance of Long-term Debt									49.9	249.4	200.2
Repayment of Long-term Debt									(185.0)	(215.0)	(360.0)
Proceeds from Issuance of Common Stock, Net of Transaction Costs									127.5	0.0	0.0
Dividends and Dividend Equivalents Paid									(67.8)	(56.4)	(49.5)
Proceeds from Shares Issued under Employee Stock Purchase Plan									1.6	0.0	0.0
Cash Exercise of Stock Options									2.4	0.9	4.0
Net Cash Provided (Used) by Financing Activities									(71.4)	(21.1)	(205.3)
Increase (Decrease) in Cash									58.9	(5.5)	(7.3)
Cash, Beginning of Year				$ 2.9				$ 8.4	2.9	8.4	15.7
Cash, End of Year	$ 61.8				$ 2.9				$ 61.8	$ 2.9	$ 8.4