Investments Net Investment Income (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Gain (Loss) on Investments [Line Items]
Gross investment income		$ 96.7	$ 105.1	$ 290.0	$ 291.4
Investment expenses	[1]	0.9	0.8	2.9	2.5
Net investment income		95.7	104.2	287.0	288.9
Net realized gains (losses)		4.7	26.9	37.7	50.0
Total realized gains (losses)		7.2	41.4	58.1	77.0
Income taxes (credits)		2.5	14.5	20.3	26.9
Interest expense incurred on funds held		0.1	0.0	0.3	0.2
Unrealized Gain (Loss) on Investments		0.1	(123.7)	350.1	(222.5)
Equity Securities [Member]
Gain (Loss) on Investments [Line Items]
Equity securities, gross realized gains		16.5	40.0	84.8	69.7
Equity securities, gross realized losses		(1.5)	(1.4)	(25.4)	(3.1)
Equity securities, net realized gains (losses)		14.9	38.5	59.4	66.6
Fixed Maturity Securities [Member]
Gain (Loss) on Investments [Line Items]
Fixed maturity securities, gross realized gains		3.9	3.1	11.0	11.3
Fixed maturity securities, gross realized losses		(11.6)	0.0	(12.3)	(0.4)
Net realized gains (losses)		(7.6)	3.1	(1.2)	10.8
Changes in unrealized investment gains (losses) on, fixed maturity securities		16.9	(41.3)	308.7	(97.3)
Less: Deferred income taxes (credits)		5.9	(14.3)	107.9	(34.0)
Net changes in unrealized investment gains (losses)		11.0	(27.0)	200.8	(63.3)
Equity securities & other long-term investments [Member]
Gain (Loss) on Investments [Line Items]
Changes in unrealized investment gains (losses) on, fixed maturity securities		(16.6)	(148.7)	229.6	(244.8)
Less: Deferred income taxes (credits)		(5.8)	(51.9)	80.3	(85.7)
Net changes in unrealized investment gains (losses)		(10.8)	(96.7)	149.2	(159.1)
Other Investments
Gain (Loss) on Investments [Line Items]
Other long-term investments, net		0.0	(0.2)	0.0	(0.5)
Fixed Maturity Securities [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income		73.2	73.8	222.7	221.4
Equity Securities [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income		22.0	30.3	62.7	66.7
Short-term Investments [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income		0.4	0.2	1.6	0.6
Investments [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income		$ 0.9	$ 0.7	$ 2.9	$ 2.6

[1] Investment expenses consist of personnel costs and investment management and custody service fees, as well as interest incurred on funds held of $.1 and $- for the quarters ended September 30, 2016 and 2015, and $.3 and $.2 for the nine months ended September 30, 2016 and 2015, respectively.