Acquisitions, Goodwill and Indefinite-Lived Intangible Assets (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2021	Dec. 31, 2019
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, Impairment Loss	$ 44.7
Impairment of Intangible Assets, Indefinite-lived (Excluding Goodwill)	$ 1.3
Goodwill		$ 0.0	$ 44.7
Indefinite-lived intangibles		$ 0.0
Weighted average period to amortize acquired intangible assets		13 years
Commercial Accounts business unit [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill			2.1
Aerospace & Programs business unit [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill			9.7
Commercial Auto business units [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill			21.3
E&S Casualty business unit [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill			6.3
Public Entity Component [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill			2.6
E & S Package component [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill			3.7
Personal Lines Business Unit [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill			$ 5.3