Discontinued Operations (Schedule of Effects of Reinsurance) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Property and casualty premiums written, Direct	$ 5,131.4	$ 5,090.8	$ 4,816.0
Property and casualty premiums written, Assumed	23.1	27.7	27.6
Property and casualty premiums written, Ceded	(556.0)	(536.8)	(458.8)
Property and casualty premiums written, Net	4,598.5	4,581.7	4,384.8
Property and casualty premiums earned, Direct	5,063.8	4,953.3	4,673.6
Property and casualty premiums earned, Assumed	24.6	26.3	26.6
Property and casualty premiums earned, Ceded	(561.0)	(505.1)	(445.8)
Premiums Earned, Net, Total	4,527.4	4,474.5	4,254.4
Property and casualty losses and LAE, Direct	3,200.5	3,223.3	2,986.2
Property and casualty losses and LAE, Assumed	15.9	21.4	25.2
Property and casualty losses and LAE, Ceded	$ (371.2)	$ (379.2)	(286.8)
Chaucer [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Property and casualty premiums written, Direct			625.3
Property and casualty premiums written, Assumed			685.3
Property and casualty premiums written, Ceded			(459.9)
Property and casualty premiums written, Net			850.7
Property and casualty premiums earned, Direct			611.9
Property and casualty premiums earned, Assumed			667.2
Property and casualty premiums earned, Ceded			(429.1)
Premiums Earned, Net, Total			850.0
Property and casualty losses and LAE, Direct			362.2
Property and casualty losses and LAE, Assumed			568.7
Property and casualty losses and LAE, Ceded			(415.4)
Policyholder Benefits and Claims Incurred, Net, Total			$ 515.5