Investments (Narrative) (Details) - USD ($) $ in Millions		3 Months Ended
	Jan. 01, 2018	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Schedule Of Investments [Line Items]
Net unrealized gains on impaired securities		$ 9.1		$ 11.5
Restricted assets, fixed maturities		528.9
Restricted assets, cash and cash equivalents		5.3
Other-than-temporary impairments on fixed maturities		1.0
Other-than-temporary impairments on fixed maturities recorded as unrealized losses in AOCI		0.3
Other-than-temporary impairments on fixed maturities recognized in earnings		0.7	$ 1.4
Equity securities, at fair value		550.9		576.5
Equity securities, cost				$ 433.7
Net unrealized losses recognized during the period on equity securities still held		$ (23.5)
Proceeds from the sale of equities			13.6
Retained Earnings [Member]
Schedule Of Investments [Line Items]
Net realized gains from sales of equity securities			2.4
Accumulated Other Comprehensive Income (Loss), net of tax [Member]
Schedule Of Investments [Line Items]
Net unrealized losses recognized during the period on equity securities still held			$ 21.5
Equity Securities [Member] | Retained Earnings [Member]
Schedule Of Investments [Line Items]
Cumulative effect on retained earnings, before tax	$ 142.8