Summary of Significant Accounting Policies Investments (Details) $ in Millions		3 Months Ended	12 Months Ended
		Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed maturity securities, amortized cost			$ 9,897.6	$ 8,537.3
Fixed maturity securities (at fair value) (amortized cost: $9,897.6 and $8,537.3)			10,496.8	8,796.5
Debt Securities, Held-to-maturity			0.0	1,021.7
Debt Securities, Held-to-maturity, Fair Value			0.0	1,058.2
Fixed maturity securities, due in one year or less, amortized cost			971.5
Fixed maturity securities, due after one year through five years, amortized cost			5,641.5
Fixed maturity securities, due after five years through ten years, amortized cost			3,108.0
Fixed maturity securities, due after ten years, amortized cost			176.5
Fixed maturity securities, due in one year or less, estimated fair value			981.6
Fixed maturity securities, due after one year through five years, estimated fair value			5,968.4
Fixed maturity securities, due after five years through ten years, estimated fair value			3,366.5
Fixed maturity securities, due after ten years, estimated fair value			180.2
Bonds on deposit with Governmental Authorities			929.3
Other than temporary impairment losses, Available-for-sale Securities				0.0	$ 0.0
Equity securities, adjusted cost			3,269.7	3,089.1
Equity securities (at fair value) (cost: $3,269.7 and $3,089.1)			4,054.8	4,030.5
Unrealized from changes in fair value of equity securities			(156.2)	599.5	(293.8)
Gain (Loss) on Investments			(142.0)	636.1	(235.6)
Short-term investments			749.6	484.3
Gross investment income			445.6	457.7	438.1
Investment expenses	[1]		6.6	6.9	6.2
Net investment income			438.9	450.7	431.8
Net realized gains (losses)			(112.1)	502.2	(185.9)
Gain (Loss) on Investments, Excluding Other than Temporary Impairments			14.2	38.6	58.2
Total realized and unrealized investment gains (losses)			14.2	36.6	58.2
Other than Temporary Impairment Losses, Investments			0.0	(2.0)	0.0
Income taxes (credits)			(29.8)	133.8	(49.6)
Changes in unrealized investment gains (losses) on fixed maturity securities:			342.7	363.8	(223.2)
Deferred income taxes (credits)			72.3	76.6	(46.9)
Net changes in unrealized investment gains (losses)			270.3	287.2	(176.3)
Net unrealized gains (losses) on securities, net of tax			270.3	287.2	(176.3)
Realized from impairments			0.0	(2.0)	0.0
Total Common Shareholders' Equity			6,186.6	6,000.1
Retained Earnings [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total Common Shareholders' Equity			4,394.8	4,386.0	3,849.8	$ 3,206.9
Cumulative Effect, Period of Adoption, Adjustment | Retained Earnings [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total Common Shareholders' Equity	[2]			(2.3)	18.4	$ 502.1
Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income			289.8	300.3	299.2
Equity Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income			149.8	141.3	124.0
Short-term Investments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income			2.2	10.1	9.8
Investments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income			$ 3.5	$ 5.8	4.9
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			74	54
Securities in unrealized loss positions, number of positions			77	101
Number of securities in continuous unrealized loss position for 12 months or greater			3	47
Fixed Maturity Securities, U S Canadian Governments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed maturity securities, amortized cost			$ 1,967.1	$ 1,842.3
Fixed maturity securities, gross unrealized gains			96.4	36.9
Fixed maturity securities, gross unrealized losses			0.3	0.4
Fixed maturity securities (at fair value) (amortized cost: $9,897.6 and $8,537.3)			2,063.2	1,878.8
Fixed maturity securities and equity, less than twelve months, fair value			416.4	217.2
Fixed maturity securities and equity, less than 12 months, unrealized losses			0.3	0.3
Fixed maturity securities and equity, 12 months or greater, fair value			0.0	53.0
Fixed maturity securities and equity, 12 months or greater, unrealized losses			0.0	0.1
Fixed maturity securities and equity, total, fair value			416.4	270.3
Fixed maturity securities and equity, total, unrealized losses			0.3	0.4
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed maturity securities, amortized cost			997.1
Fixed maturity securities, gross unrealized gains			66.3
Fixed maturity securities, gross unrealized losses			0.0
Fixed maturity securities (at fair value) (amortized cost: $9,897.6 and $8,537.3)			1,063.5
Debt Securities, Held-to-maturity				1,021.7
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain				36.5
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss				0.0
Debt Securities, Held-to-maturity, Fair Value				1,058.2
Fixed maturity securities and equity, less than twelve months, fair value				0.0
Fixed maturity securities and equity, less than 12 months, unrealized losses				0.0
Fixed maturity securities and equity, 12 months or greater, fair value				21.7
Fixed maturity securities and equity, 12 months or greater, unrealized losses				0.0
Fixed maturity securities and equity, total, fair value				21.7
Fixed maturity securities and equity, total, unrealized losses				$ 0.0
Net unrealized gains (losses) on securities, net of tax		$ 48.5
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
Securities in unrealized loss positions, number of positions				8
Number of securities in continuous unrealized loss position for 12 months or greater				8
Corporate Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed maturity securities, amortized cost			6,933.3	$ 6,694.9
Fixed maturity securities, gross unrealized gains			440.1	225.5
Fixed maturity securities, gross unrealized losses			3.4	2.8
Fixed maturity securities (at fair value) (amortized cost: $9,897.6 and $8,537.3)			7,370.0	6,917.6
Fixed maturity securities and equity, less than twelve months, fair value			333.6	176.4
Fixed maturity securities and equity, less than 12 months, unrealized losses			3.4	1.9
Fixed maturity securities and equity, 12 months or greater, fair value			0.0	54.3
Fixed maturity securities and equity, 12 months or greater, unrealized losses			0.0	0.8
Fixed maturity securities and equity, total, fair value			333.6	230.7
Fixed maturity securities and equity, total, unrealized losses			3.4	2.8
Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed maturity securities, amortized cost			9,897.6	8,537.3
Fixed maturity securities, gross unrealized gains			602.9	262.5
Fixed maturity securities, gross unrealized losses			3.8	3.3
Fixed maturity securities (at fair value) (amortized cost: $9,897.6 and $8,537.3)			10,496.8	8,796.5
Fixed maturity securities and equity, less than twelve months, fair value			750.0	393.7
Fixed maturity securities and equity, less than 12 months, unrealized losses			3.8	2.3
Fixed maturity securities and equity, 12 months or greater, fair value			0.0	107.4
Fixed maturity securities and equity, 12 months or greater, unrealized losses			0.0	1.0
Fixed maturity securities and equity, total, fair value			750.0	501.1
Fixed maturity securities and equity, total, unrealized losses			3.8	3.3
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			(130.9)	586.9	(244.8)
Equity securities, adjusted cost			3,269.7	3,089.1
Equity securities, gross unrealized gains			1,028.1	968.0
Equity securities, gross unrealized losses			(243.0)	(26.6)
Equity securities (at fair value) (cost: $3,269.7 and $3,089.1)			4,054.8	4,030.5
Fair Value, Inputs, Level 1 [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Short-term investments			749.6	484.3
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: $9,897.6 and $8,537.3)			1,262.2	1,068.1
Fair Value, Inputs, Level 1 [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed maturity securities (at fair value) (amortized cost: $9,897.6 and $8,537.3)			0.0
Debt Securities, Held-to-maturity				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: $9,897.6 and $8,537.3)			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: $3,269.7 and $3,089.1)			4,052.9	4,028.7
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: $9,897.6 and $8,537.3)			801.0	810.7
Fair Value, Inputs, Level 2 [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed maturity securities (at fair value) (amortized cost: $9,897.6 and $8,537.3)			1,063.5
Debt Securities, Held-to-maturity				1,058.2
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: $9,897.6 and $8,537.3)			7,359.5	6,907.1
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: $3,269.7 and $3,089.1)			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: $9,897.6 and $8,537.3)			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]
Fixed maturity securities (at fair value) (amortized cost: $9,897.6 and $8,537.3)			0.0
Debt Securities, Held-to-maturity				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: $9,897.6 and $8,537.3)			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: $3,269.7 and $3,089.1)			1.8	1.7
Other Long-term Investments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Net realized gains (losses)			0.3	(2.5)	1.6
Equity Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Realized gains on securities			22.5	153.1	71.9
Realized losses on securities			(1.2)	(109.9)	(10.4)
Net realized gains (losses)			21.3	43.2	61.4
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:			3.2	1.2	(1.3)
Deferred income taxes (credits)			0.6	0.2	(0.2)
Net changes in unrealized investment gains (losses)			2.5	1.0	(1.0)
Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Realized gains on securities			10.9	9.7	2.4
Realized losses on securities			(18.4)	(11.7)	(7.2)
Net realized gains (losses)			(7.4)	(1.9)	(4.8)
Changes in unrealized investment gains (losses) on fixed maturity securities:			339.4	362.6	(221.9)
Deferred income taxes (credits)			71.6	76.3	(46.6)
Net changes in unrealized investment gains (losses)			$ 267.7	$ 286.2	$ (175.2)

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

[2] Reflects the Company's adoption of new accounting principles relating to credit losses, lease accounting, and equity securities effective January 1, 2020, 2019 and 2018, respectively. Refer to additional discussion in Notes 1 and 4 to the Consolidated Financial Statements.