Segment Information (Identifiable Assets by Business Segment) (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Segment Reporting Information [Line Items]
Identifiable assets	$ 14,594.1	$ 14,612.6
Identifiable assets	85.4	86.6
Property and Casualty [Member]
Segment Reporting Information [Line Items]
Identifiable assets	14,508.7	14,526.0
Assets of Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Identifiable assets	$ 85.4	$ 86.6