Segment Information (Identifiable Assets by Business Segment) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Identifiable assets	$ 13,448.0	$ 13,443.7
Identifiable assets	99.6	110.2
Property and Casualty Insurance Companies [Member]
Segment Reporting Information [Line Items]
Identifiable assets	13,348.4	13,333.5
Assets of Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Identifiable assets	$ 99.6	$ 110.2