Reinsurance (Narrative) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effects of Reinsurance [Line Items]
Property and casualty premiums earned, Ceded		$ 445.8	$ 443.1	$ 397.5
Property and casualty losses and LAE, Ceded	[1]	286.8	375.5	269.7
Reinsurance recoverable on paid and unpaid losses and unearned premiums		1,648.6	1,625.5
MCCA [Member]
Effects of Reinsurance [Line Items]
Property and casualty premiums earned, Ceded		70.9	62.8	58.6
Property and casualty losses and LAE, Ceded		108.8	81.4	$ 75.0
Reinsurance recoverable on paid and unpaid losses and unearned premiums		$ 977.1	$ 930.6
MCCA [Member] | Reinsurance Receivable [Member] | Reinsurer Concentration Risk [Member]
Effects of Reinsurance [Line Items]
Percentage of reinsurance receivable represented by segment		59.30%

[1] The higher level of ceded losses and LAE in 2017 as compared to 2016 and 2018 is primarily due to higher non-catastrophe large loss activity occurring in certain domestic lines