Schedule VI - Insurance Operations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule III - Insurance Information
Deferred acquisition costs	$ 141.6	$ 120.9	$ 84.7
Loss and loss adjustment expense reserves	2,016.7	1,898.5	1,620.1
Unearned insurance and reinsurance premiums	647.2	506.8	398.0
Net earned insurance and reinsurance premiums	1,262.3	1,035.3	890.1
Net investment income	71.4	56.8	56.2
Loss and loss expenses incurred related to current year	907.3	811.8	583.0
Loss and loss expenses incurred related to prior year	(7.3)	(0.6)	(63.7)
Net paid losses and loss expenses	791.7	612.3	548.3
Insurance and reinsurance acquisition costs	255.4	197.2	210.3
Net premiums written	$ 1,357.1	$ 1,090.2	$ 938.1