Business Segments - Segment Assets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Total Assets	$ 8,376.2	$ 8,210.5
Property and Casualty Insurance [Member]
Segment Reporting Information [Line Items]
Total Assets	2,894.9	2,815.1
Life and Health Insurance [Member]
Segment Reporting Information [Line Items]
Total Assets	5,079.3	4,888.7
Corporate and Other, Net [Member]
Segment Reporting Information [Line Items]
Total Assets	$ 402.0	$ 506.7