Reinsurance (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Effects of Reinsurance [Line Items]
Property and casualty premiums earned, Ceded	$ 561.0	$ 505.1	$ 445.8
Property and casualty losses and LAE, Ceded	371.2	379.2	286.8
Reinsurance recoverable on paid and unpaid losses and unearned premiums	1,874.3	1,814.0
MCCA [Member]
Effects of Reinsurance [Line Items]
Property and casualty premiums earned, Ceded	84.6	84.3	70.9
Property and casualty losses and LAE, Ceded	99.2	108.5	$ 108.8
Reinsurance recoverable on paid and unpaid losses and unearned premiums	$ 1,024.7	$ 1,023.7
MCCA [Member] \| Reinsurance Receivable [Member] \| Reinsurer Concentration Risk [Member]
Effects of Reinsurance [Line Items]
Percentage of reinsurance receivable represented by segment	54.70%

Amount of earned premiums ceded to other entities.

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