Business Segments - Segment Assets (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Total Assets	$ 8,210.5	$ 8,036.1
Property and Casualty Insurance [Member]
Segment Reporting Information [Line Items]
Total Assets	2,815.1	2,749.0
Life and Health Insurance [Member]
Segment Reporting Information [Line Items]
Total Assets	4,888.7	4,733.9
Corporate and Other, Net [Member]
Segment Reporting Information [Line Items]
Total Assets	$ 506.7	$ 553.2