Investment Income and Gains and Losses (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Gain (Loss) on Investments [Line Items]
Net unrealized gains recognized during the period on equity securities still held	$ (39.5)
Net other-than-temporary impairment losses on securities recognized in earnings	4.6	$ 5.6	$ 27.4
Other-than-temporary impairments from continuing operations	5.4	5.9	20.6
Other-than-temporary impairments recorded as unrealized losses in AOCI	0.8	0.3	6.8
Proceeds from sale of equity securities	128.8	245.0
Gross realized gains on sale of equity securities	18.8	31.1
Fixed Maturities [Member]
Gain (Loss) on Investments [Line Items]
Net other-than-temporary impairment losses on securities recognized in earnings	2.6	1.8	16.0
Other Invested Assets [Member]
Gain (Loss) on Investments [Line Items]
Net other-than-temporary impairment losses on securities recognized in earnings	2.0	2.0
Equity Securities And Other [Member]
Gain (Loss) on Investments [Line Items]
Net other-than-temporary impairment losses on securities recognized in earnings		1.8	2.7
Credit Impairments
Gain (Loss) on Investments [Line Items]
Net other-than-temporary impairment losses on securities recognized in earnings			8.4
Accumulated Other Comprehensive Income (Loss), net of tax [Member]
Gain (Loss) on Investments [Line Items]
Net unrealized gains recognized during the period on equity securities still held		$ 56.8	$ 37.8
ASC No. 2016-01 [Member]
Gain (Loss) on Investments [Line Items]
Net unrealized gains recognized during the period on equity securities still held	$ 5.3