SCHEDULE V VALUATION AND QUALIFYING ACCOUNTS (Parenthetical) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Valuation And Qualifying Accounts Disclosure [Line Items]
Valuation allowance, balance reduced as part of sale of business	[1]	$ (6.4)	$ 0.1		$ 0.1
Valuation allowance, balance		8.1	14.0		16.8		$ 12.9
Allowance for uncollectible reinsurance recoverables [Member]
Valuation And Qualifying Accounts Disclosure [Line Items]
Valuation allowance, balance reduced as part of sale of business	[1]	(6.4)	0.1	[2]	0.1	[2],[3]
Valuation allowance, balance		3.9	9.7	[2]	13.1	[2],[3]	9.7	[2],[3]
Chaucer [Member] | Allowance for uncollectible reinsurance recoverables [Member]
Valuation And Qualifying Accounts Disclosure [Line Items]
Valuation allowance, balance reduced as part of sale of business		$ 6.4
Valuation allowance, balance			$ 6.4		$ 6.3		$ 6.2

[1] Amounts charged to other accounts include foreign exchange gains and losses. Additionally, in 2018 the Company sold its Chaucer business; and accordingly, the $6.4 million balance associated with this business was reduced as part of the sale.

[2] The balance at beginning of period and the balance at end of period include $6.3 million and $6.4 million, respectively, for the year-ended 2017 and $6.2 million and $6.3 million, respectively, for the year-ended 2016 related to the former Chaucer business.

[3] Activity primarily relates to the impairment in 2016 and subsequent write-off in 2017 of a receivable that is associated with a single reinsurance counterparty that was placed into conservation by the state of California in July 2016. This counterparty is not involved in the Company’s ongoing reinsurance program and the entire receivable related to this counterparty resulted from a one-time block reinsurance transaction that occurred in 2012 as part of the Company’s exposure management actions.