Schedule 2 - Parent Company Financial Statements - Statements of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Activities:
Net Income	$ 6.5	$ 92.2	$ 37.6	$ 53.8	$ 36.9	$ 47.7	$ 36.6	$ (0.3)	$ 190.1	$ 120.9	$ 16.8
Adjustment Required to Reconcile Net Income to Net Cash Provided by Operations:
Equity Securities, FV-NI, Gain (Loss)	76.4	(11.0)	(0.4)	(0.7)					64.3	0.0	0.0
Debt and Equity Securities, Realized Gain (Loss)	16.4	3.6	3.8	2.6	11.5	8.1	26.4	10.5	26.4	56.5	33.1
Net Impairment Losses Recognized in Earnings	2.2	$ 1.8	$ 0.0	0.5	3.8	$ 2.9	$ 2.6	5.0	4.5	14.3	32.7
Other, Net									8.6	(2.1)	0.7
Net Cash Provided by Operating Activities									539.2	240.6	240.5
Investing Activities:
Sales, Paydowns and Maturities of Fixed Maturities									2,643.3	528.2	532.3
Purchases of Fixed Maturities									(2,413.2)	(710.1)	(850.3)
Sales of Equity Securities									351.9	342.0	158.9
Sales of Fair Value Option Investments									0.0	42.2	72.2
Payments to Acquire Available-for-sale Securities, Equity									478.5	342.7	90.1
Purchases of Fair Value Option Investments									0.0	(7.0)	(21.0)
Payments to Acquire Businesses, Net of Cash Acquired									(560.6)	0.0	0.0
Change in Short-term Investments									52.7	39.4	(18.0)
Net Cash Used by Investing Activities									(497.6)	(105.4)	(238.1)
Notes Payable Proceeds:
Cash Dividends Paid									(56.4)	(49.5)	(49.2)
Common Stock Repurchases									0.0	0.0	(3.8)
Cash Exercise of Stock Options									0.9	4.0	3.5
Net Cash Used by Financing Activities									(12.2)	(205.2)	(48.4)
Decrease in Cash									29.4	(70.0)	(46.0)
Cash, Beginning of Year				45.7				115.7	45.7	115.7	161.7
Cash, End of Year	75.1				45.7				75.1	45.7	115.7	$ 161.7
Parent Company [Member]
Operating Activities:
Net Income									190.1	120.9	16.8
Adjustment Required to Reconcile Net Income to Net Cash Provided by Operations:
Equity in Net Income of Subsidiaries									(238.6)	(127.6)	(40.4)
Cash Dividends from Subsidiaries									130.4	108.1	77.7
Cash Received for Benefit Plan from Subsidiary									0.0	0.0	1.4
Cash Contribution to Defined Benefit Plan									(5.0)	0.0	(9.0)
Equity Securities, FV-NI, Gain (Loss)									(1.4)	0.0	0.0
Debt and Equity Securities, Realized Gain (Loss)									(0.7)	0.6	0.1
Other, Net									29.6	0.7	(9.3)
Net Cash Provided by Operating Activities									105.8	101.5	37.1
Investing Activities:
Capital Contributed to Subsidiary									(20.0)	0.0	(52.9)
Sales, Paydowns and Maturities of Fixed Maturities									(0.2)	45.7	73.5
Purchases of Fixed Maturities									0.0	0.0	(77.9)
Sales of Equity Securities									67.5	0.0	3.5
Sales of Fair Value Option Investments									0.0	42.2	72.2
Payments to Acquire Available-for-sale Securities, Equity									2.3	0.0	0.0
Purchases of Fair Value Option Investments									0.0	(7.0)	(21.0)
Payments to Acquire Businesses, Net of Cash Acquired									(564.6)	0.0	0.0
Change in Short-term Investments									253.4	15.6	14.8
Net Cash Used by Investing Activities									(90.2)	96.5	12.2
Notes Payable Proceeds:
Net Proceeds from Issuance of Subordinated Debentures									249.4	200.2	0.0
Repayments of Debt									(215.0)	(360.0)	0.0
Cash Dividends Paid									(56.4)	(49.5)	(49.2)
Common Stock Repurchases									0.0	0.0	(3.8)
Cash Exercise of Stock Options									0.9	4.0	3.5
Net Cash Used by Financing Activities									(21.1)	(205.3)	(49.5)
Decrease in Cash									(5.5)	(7.3)	(0.2)
Cash, Beginning of Year				$ 8.4				$ 15.7	8.4	15.7	15.9
Cash, End of Year	$ 2.9				$ 8.4				2.9	8.4	$ 15.7	15.9
Capital Distribution from Subsidiary									$ 176.0	$ 0.0		$ 0.0