Discontinued Chaucer Business (Summary of Carrying Amounts of Major Assets and Liabilities Reflected in Consolidated Balance Sheets) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Assets
Investments	$ 1,923.4	$ 2,153.5
Cash and cash equivalents	133.4	78.5
Accrued investment income	12.7	14.5
Premiums and accounts receivable, net	558.3	471.9
Deferred acquisition costs	121.4	120.2
Reinsurance recoverable on paid and unpaid losses and unearned premiums	1,278.8	1,431.5
Goodwill	13.0	13.8
Other assets	206.2	182.7
Total assets held-for-sale	4,247.2	4,466.6
Liabilities
Loss and loss adjustment expense reserves	2,472.5	2,686.5
Unearned premiums	706.7	631.9
Reinsurance premium payable	375.7	293.3
Other liabilities	105.5	95.5
Total liabilities held-for-sale	$ 3,660.4	$ 3,707.2