Investments (Narrative) (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
	Jan. 02, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Schedule Of Investments [Line Items]
Net unrealized gains on impaired securities		$ 9.1		$ 9.1		$ 11.0
Other-than-temporary impairments on fixed maturities		0.7	$ 1.5	3.4	$ 4.9
Other-than-temporary impairments on fixed maturities recorded as unrealized losses in AOCI		0.3	0.2	0.6	0.4
Other-than-temporary impairments on fixed maturities recognized in earnings		0.4	1.3	2.8	4.5
Equity securities, at fair value		574.8		574.8		576.2
Equity securities, cost						$ 433.7
Net unrealized gains recognized during the period on equity securities still held		23.8		7.7
Proceeds from the sale of equities			68.2		114.5
Retained Earnings [Member]
Schedule Of Investments [Line Items]
Net realized gains from sales of equity securities			11.8		17.6
Accumulated Other Comprehensive Income (Loss), net of tax [Member]
Schedule Of Investments [Line Items]
Net unrealized gains recognized during the period on equity securities still held			7.9		37.2
Equity Securities [Member] | Retained Earnings [Member]
Schedule Of Investments [Line Items]
Cumulative effect on retained earnings, before tax	$ 142.5
Chaucer [Member]
Schedule Of Investments [Line Items]
Other-than-temporary impairments from discontinued operations		$ 0.2	$ 0.1	$ 0.4	$ 0.1