Discontinued Chaucer Business (Summary of Carrying Amounts of Major Assets and Liabilities Reflected in Consolidated Balance Sheets) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Assets
Investments	$ 24.5	$ 2,153.5
Cash and cash equivalents	3.3	78.5
Premiums and accounts receivable, net	2.8	471.9
Deferred acquisition costs	1.4	120.2
Reinsurance recoverable on paid and unpaid losses and unearned premiums	2.3	1,431.5
Goodwill	6.5	13.8
Other assets	16.6	197.2
Total assets	57.4	4,466.6
Liabilities
Loss and loss adjustment expense reserves	2.0	2,686.5
Unearned premiums	7.1	631.9
Reinsurance premium payable	1.5	293.3
Other liabilities	11.6	95.5
Total liabilities	$ 22.2	$ 3,707.2