Condensed Consolidated Statements of Cash Flows - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2019
Statement of Cash Flows [Abstract]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 301.3	$ 107.0	$ 75.1	$ 136.8
Operating Activities:
Net Income	64.0	155.3
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities:
Increase in Deferred Policy Acquisition Costs	(13.8)	(29.2)
Amortization of Intangible Assets Acquired	4.7	8.7
Equity in Earnings of Equity Method Limited Liability Investments	(1.8)	(3.6)
Distribution of Accumulated Earnings of Equity Method Limited Liability Investments	1.1	4.3
Decrease (Increase) in Value of Equity and Convertible Securities at Fair Value	117.8	(64.4)
Amortization of Investment Securities and Depreciation of Investment Real Estate	0.4	1.6
Net Realized Gains on Sales of Investments	(16.5)	(16.1)
Impairment Losses	12.0	3.6
Depreciation and Amortization of Property, Equipment and Software	8.0	3.1
Increase in Receivables	(89.4)	(56.1)
Increase (Decrease) in Insurance Reserves	(29.4)	3.8
Increase in Unearned Premiums	75.9	75.2
Change in Income Taxes	14.7	40.1
Change in Accrued Expenses and Other Liabilities	(76.3)	(42.4)
Other, Net	(9.1)	5.1
Net Cash Provided by Operating Activities	62.3	89.0
Investing Activities:
Sales, Paydowns and Maturities of Fixed Maturities	225.5	402.6
Purchases of Fixed Maturities	(393.9)	(368.4)
Sales of Equity and Convertible Securities	372.1	39.6
Purchases of Equity and Convertible Securities	(290.0)	(207.6)
Acquisition and Improvements of Investment Real Estate	0.1	0.4
Sales of Investment Real Estate	1.9		0.0
Sale of and Return of Investment of Equity Method Limited Liability Investments	4.0	7.1
Acquisitions of Equity Method Limited Liability Investments	(9.2)	(18.6)
Decrease (Increase) in Short-term Investments	301.8	(53.3)
Acquisition of Mortgage Loans	4.6		0.0
Paydowns of Mortgage Loans	8.5		0.0
Increase in Other Investments	(0.4)	(10.6)
Purchases of Corporate-owned Life Insurance	(100.0)	0.0
Acquisition of Software and Long-lived Assets	(20.1)	(9.3)
Other, Net	(1.5)	(0.8)
Net Cash Provided (Used) by Investing Activities	94.0	(219.7)
Financing Activities:
Proceeds from Policyholder Contract Liabilities	156.6	187.7
Repayment of Policyholder Contract Liabilities	(36.2)	(10.0)
Common Stock Repurchases	(95.9)		$ 0.0
Dividends and Dividend Equivalents Paid	(20.0)	(16.4)
Proceeds from Shares Issued under Employee Stock Purchase Plan	1.0	0.0
Cash Exercise of Stock Options	2.7	1.3
Net Cash Provided by Financing Activities	8.2	162.6
Increase in Cash	164.5	$ 31.9
Cash, Beginning of Year	136.8
Cash, End of Period	$ 301.3