Summary of Significant Accounting Policies Investments (Details) $ in Millions		12 Months Ended
		Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Other-than-temporary impairments, minimum percentage decline			20.00%
Fixed maturity securities, amortized cost		$ 8,537.3	$ 8,285.0
Fixed maturity securities (at fair value) (amortized cost: $8,537.3 and $8,285.0)		8,796.5	8,182.8
Debt Securities, Held-to-maturity		1,021.7	1,044.8
Debt Securities, Held-to-maturity, Fair Value		1,058.2	1,034.5
Fixed maturity securities, due in one year or less, amortized cost		1,018.3
Fixed maturity securities, due after one year through five years, amortized cost		4,931.5
Fixed maturity securities, due after five years through ten years, amortized cost		2,548.7
Fixed maturity securities, due after ten years, amortized cost		38.5
Fixed maturity securities, due in one year or less, estimated fair value		1,022.3
Fixed maturity securities, due after one year through five years, estimated fair value		5,057.3
Fixed maturity securities, due after five years through ten years, estimated fair value		2,676.6
Fixed maturity securities, due after ten years, estimated fair value		40.0
Bonds on deposit with Governmental Authorities		917.5
Other than temporary impairment losses, Available-for-sale Securities		2.0	0.0	$ 0.0
Equity securities, adjusted cost		3,089.1	3,039.1
Equity securities (at fair value) (cost: $3,089.1 and $3,039.1)		4,030.5	3,380.9
Reclassification - change in accounting principle			502.1
Unrealized from changes in fair value of equity securities		599.5	(293.8)	0.0
Short-term investments		484.3	354.9
Gross investment income		457.7	438.1	414.1
Investment expenses	[1]	6.9	6.2	4.6
Net investment income		450.7	431.8	409.4
Net realized gains (losses)		502.2	(185.9)	242.4
Gain (Loss) on Investments, Excluding Other than Temporary Impairments		38.6	58.2	211.6
Total realized and unrealized investment gains (losses)		636.1	(235.6)	211.6
Other than Temporary Impairment Losses, Investments		(2.0)	0.0	0.0
Income taxes (credits)	[2],[3]	133.8	(49.6)	(30.8)
Changes in unrealized investment gains (losses) on fixed maturity securities:		363.8	(223.2)	113.4
Deferred income taxes (credits)		76.6	(46.9)	39.5
Net changes in unrealized investment gains (losses)		287.2	(176.3)	73.9
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost		1.0
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value		1.0
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost		380.8
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value		389.8
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost		639.8
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value		667.3
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Amortized Cost		0.0
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value		0.0
Tax reform deferred tax benefit on unrezlized gains (losses)				104.9
Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income		300.3	299.2	293.2
Equity Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income		141.3	124.0	110.9
Short-term Investments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income		10.1	9.8	5.4
Investments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income		$ 5.8	$ 4.9	4.5
Available-for-sale Securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Number of securities in continuous unrealized loss position for less than 12 months		54	760
Securities in unrealized loss positions, number of positions		101	1,095
Number of securities in continuous unrealized loss position for 12 months or greater		47	335
Fixed Maturity Securities, U S Canadian Governments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed maturity securities, amortized cost		$ 1,842.3	$ 1,535.3
Fixed maturity securities, gross unrealized gains		36.9	5.7
Fixed maturity securities, gross unrealized losses		0.4	16.5
Fixed maturity securities (at fair value) (amortized cost: $8,537.3 and $8,285.0)		1,878.8	1,524.4
Fixed maturity securities and equity, less than twelve months, fair value		217.2	616.7
Fixed maturity securities and equity, less than 12 months, unrealized losses		0.3	8.4
Fixed maturity securities and equity, 12 months or greater, fair value		53.0	487.1
Fixed maturity securities and equity, 12 months or greater, unrealized losses		0.1	8.1
Fixed maturity securities and equity, total, fair value		270.3	1,103.9
Fixed maturity securities and equity, total, unrealized losses		0.4	16.5
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Debt Securities, Held-to-maturity		1,021.7	1,044.8
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain		36.5	3.5
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss		0.0	13.7
Debt Securities, Held-to-maturity, Fair Value		1,058.2	1,034.5
Fixed maturity securities and equity, less than twelve months, fair value		0.0	271.9
Fixed maturity securities and equity, less than 12 months, unrealized losses		0.0	2.3
Fixed maturity securities and equity, 12 months or greater, fair value		21.7	407.7
Fixed maturity securities and equity, 12 months or greater, unrealized losses		0.0	11.4
Fixed maturity securities and equity, total, fair value		21.7	679.7
Fixed maturity securities and equity, total, unrealized losses		$ 0.0	$ 13.7
Held-to-maturity Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Number of securities in continuous unrealized loss position for less than 12 months		0	94
Securities in unrealized loss positions, number of positions		8	239
Number of securities in continuous unrealized loss position for 12 months or greater		8	145
Corporate Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed maturity securities, amortized cost		$ 6,694.9	$ 6,749.6
Fixed maturity securities, gross unrealized gains		225.5	31.4
Fixed maturity securities, gross unrealized losses		2.8	122.7
Fixed maturity securities (at fair value) (amortized cost: $8,537.3 and $8,285.0)		6,917.6	6,658.3
Fixed maturity securities and equity, less than twelve months, fair value		176.4	3,440.8
Fixed maturity securities and equity, less than 12 months, unrealized losses		1.9	77.9
Fixed maturity securities and equity, 12 months or greater, fair value		54.3	1,096.4
Fixed maturity securities and equity, 12 months or greater, unrealized losses		0.8	44.7
Fixed maturity securities and equity, total, fair value		230.7	4,537.3
Fixed maturity securities and equity, total, unrealized losses		2.8	122.7
Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed maturity securities, amortized cost		8,537.3	8,285.0
Fixed maturity securities, gross unrealized gains		262.5	37.1
Fixed maturity securities, gross unrealized losses		3.3	139.2
Fixed maturity securities (at fair value) (amortized cost: $8,537.3 and $8,285.0)		8,796.5	8,182.8
Fixed maturity securities and equity, less than twelve months, fair value		393.7	4,057.6
Fixed maturity securities and equity, less than 12 months, unrealized losses		2.3	86.4
Fixed maturity securities and equity, 12 months or greater, fair value		107.4	1,583.6
Fixed maturity securities and equity, 12 months or greater, unrealized losses		1.0	52.8
Fixed maturity securities and equity, total, fair value		501.1	5,641.2
Fixed maturity securities and equity, total, unrealized losses		3.3	139.2
Equity Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Net Unrealized Gains Losses Recognized from Changes in Fair Value of Equity Securities Held		586.9	(244.8)
Equity securities, adjusted cost		3,089.1	3,039.1
Equity securities, gross unrealized gains		968.0	517.3
Equity securities, gross unrealized losses		(26.6)	(175.4)
Equity securities (at fair value) (cost: $3,089.1 and $3,039.1)		4,030.5	3,380.9
Unrealized from changes in fair value of equity securities			(293.8)
Fair Value, Inputs, Level 1 [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Short-term investments		484.3	354.9
Fair Value, Inputs, Level 1 [Member] | Fixed Maturity Securities, U S Canadian Governments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed maturity securities (at fair value) (amortized cost: $8,537.3 and $8,285.0)		1,068.1	714.0
Fair Value, Inputs, Level 1 [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Debt Securities, Held-to-maturity		0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Corporate Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed maturity securities (at fair value) (amortized cost: $8,537.3 and $8,285.0)		0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Equity securities (at fair value) (cost: $3,089.1 and $3,039.1)		4,028.7	3,379.2
Fair Value, Inputs, Level 2 [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Short-term investments		0.0	0.0
Fair Value, Inputs, Level 2 [Member] | Fixed Maturity Securities, U S Canadian Governments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed maturity securities (at fair value) (amortized cost: $8,537.3 and $8,285.0)		810.7	810.3
Fair Value, Inputs, Level 2 [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Debt Securities, Held-to-maturity		1,058.2	1,034.5
Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed maturity securities (at fair value) (amortized cost: $8,537.3 and $8,285.0)		6,907.1	6,647.8
Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Equity securities (at fair value) (cost: $3,089.1 and $3,039.1)		0.0	0.0
Fair Value, Inputs, Level 3 [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Short-term investments		0.0	0.0
Fair Value, Inputs, Level 3 [Member] | Fixed Maturity Securities, U S Canadian Governments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed maturity securities (at fair value) (amortized cost: $8,537.3 and $8,285.0)		0.0	0.0
Fair Value, Inputs, Level 3 [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Debt Securities, Held-to-maturity		0.0	0.0
Fair Value, Inputs, Level 3 [Member] | Corporate Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed maturity securities (at fair value) (amortized cost: $8,537.3 and $8,285.0)		10.5	10.5
Fair Value, Inputs, Level 3 [Member] | Equity Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Equity securities (at fair value) (cost: $3,089.1 and $3,039.1)		1.7	1.7
Other Long-term Investments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Net realized gains (losses)		(2.5)	1.6	0.1
Equity Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Realized gains on securities		153.1	71.9	217.8
Realized losses on securities		(109.9)	(10.4)	(23.0)
Net realized gains (losses)		43.2	61.4	194.7
Equity securities & other long-term investments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Changes in unrealized investment gains (losses) on fixed maturity securities:		1.2	(1.3)	144.7
Deferred income taxes (credits)		0.2	(0.2)	50.2
Net changes in unrealized investment gains (losses)		1.0	(1.0)	94.4
Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Realized gains on securities		9.7	2.4	22.1
Realized losses on securities		(11.7)	(7.2)	(5.4)
Net realized gains (losses)		(1.9)	(4.8)	16.6
Changes in unrealized investment gains (losses) on fixed maturity securities:		362.6	(221.9)	(31.2)
Deferred income taxes (credits)		76.3	(46.6)	(10.7)
Net changes in unrealized investment gains (losses)		$ 286.2	$ (175.2)	$ (20.5)

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

[2] (b)    Reflects primarily the combination of fully taxable investment gains or losses.

[3] $104.9, for 2017, of deferred income tax credits to adjust to the new 21% tax rate of 2018 pertaining to unrealized gains (losses) as of December 31, 2017. Deferred income taxes on unrealized gains (losses) would normally be a part of the statement of comprehensive income rather than the income statement.