Investments Net Investment Income (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Gain (Loss) on Securities [Line Items]
Gross investment income		$ 110.6	$ 114.7	$ 226.5	$ 228.6
Investment expenses	[1]	1.9	1.7	3.6	3.4
Net investment income		108.7	113.0	222.9	225.1
Net realized gains (losses)		273.8	29.1	(471.7)	319.8
Gain (Loss) on Investments, Excluding Other than Temporary Impairments		(7.3)	12.5	11.2	24.9
Realized Investment Gains Losses From Impairment		0.0	(2.0)	0.0	(2.0)
Equity Securities, FV-NI, Unrealized Gain (Loss)		354.0	26.3	(608.7)	382.0
Total realized and unrealized investment gains (losses)		346.7	36.9	(597.4)	405.0
Current and deferred income taxes (credits)		72.8	7.7	(125.7)	85.1
Changes in unrealized investment gains (losses) on, fixed maturity securities		471.3	141.2	297.7	316.5
Less: Deferred income taxes (credits)		99.2	29.7	62.8	66.7
Unrealized Gain (Loss) on Investments		372.0	111.4	234.8	249.8
Equity Securities [Member]
Gain (Loss) on Securities [Line Items]
Equity securities, gross realized gains		1.5	13.1	21.0	54.3
Equity securities, gross realized losses		0.0	0.0	(1.2)	(28.0)
Equity securities, net realized gains (losses)		1.5	13.0	19.8	26.3
Fixed Maturity Securities [Member]
Gain (Loss) on Securities [Line Items]
Fixed maturity securities, gross realized gains		2.7	3.6	3.6	3.7
Fixed maturity securities, gross realized losses		(11.6)	(4.1)	(12.2)	(5.1)
Net realized gains (losses)		(8.8)	(0.4)	(8.5)	(1.4)
Changes in unrealized investment gains (losses) on, fixed maturity securities		467.2	141.0	294.4	315.4
Less: Deferred income taxes (credits)		98.3	29.7	62.1	66.5
Net changes in unrealized investment gains (losses)		368.8	111.3	232.2	248.9
Equity securities & other long-term investments [Member]
Gain (Loss) on Securities [Line Items]
Changes in unrealized investment gains (losses) on, fixed maturity securities		4.0	0.2	3.3	1.1
Less: Deferred income taxes (credits)		0.8	0.0	0.7	0.2
Net changes in unrealized investment gains (losses)		3.2	0.1	2.6	0.8
Other Investments
Gain (Loss) on Securities [Line Items]
Investment gains (losses) - other investments		0.0	0.0	0.0	0.1
Fixed Maturity Securities [Member]
Gain (Loss) on Securities [Line Items]
Gross investment income		72.4	75.2	147.0	149.8
Equity Securities [Member]
Gain (Loss) on Securities [Line Items]
Gross investment income		36.8	35.3	75.0	70.3
Short-term Investments [Member]
Gain (Loss) on Securities [Line Items]
Gross investment income		0.2	2.5	2.1	4.9
Investments [Member]
Gain (Loss) on Securities [Line Items]
Gross investment income		$ 1.1	$ 1.7	$ 2.3	$ 3.3

[1] (a) Investment expenses largely consist of personnel costs and investment management and custody service fees.