Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Fixed maturities, available-for-sale, amortized cost	$ 2,199.0	$ 2,173.1
Deferred policy acquisitions costs from affiliates	55.5	57.2
Losses and loss expenses payable, affiliates	677.6	711.4
Unearned premiums, affiliates	157.3	187.9
Notes payable, affiliates	15.2	15.2
Net Postretirement and Pension Benefits	(34.5)	(34.8)
Net Other Liabilities from Affiliates	$ 17.6	$ 15.5
Common stock, no par value (usd per share)	$ 0	$ 0
Common stock, shares authorized (shares)	100,000,000	100,000,000
Common stock, shares issued (shares)	49,500,000	49,200,000
Common stock, stated value per share (usd per share)	$ 2.5	$ 2.5
Treasury stock (shares)	6,800,000	6,800,000
Net Accumulated Other Comprehensive Income	$ (51.0)	$ (50.7)
Class A Preferred Stock [Member]
Preferred stock, no par value (usd per share)	$ 0	$ 0
Preferred stock, shares authorized (shares)	2,500,000	2,500,000
Preferred stock, shares issued (shares)	0	0
Class B Preferred Stock [Member]
Preferred stock, no par value (usd per share)	$ 0	$ 0
Preferred stock, shares authorized (shares)	2,500,000	2,500,000
Preferred stock, shares issued (shares)	0	0