Summary of Significant Accounting Policies Investments (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Other-than-temporary impairments, minimum percentage decline	20.00%
Realized investment gains losses from impairment	$ 0		$ 0.2		$ 50.2
Fixed maturity securities, amortized cost	8,477.3		7,993.1
Fixed maturity securities, estimated fair value	8,712.3		8,566.2
Fixed maturity securities, due in one year or less, amortized cost	785.5
Fixed maturity securities, due after one year through five years, amortized cost	3,957.0
Fixed maturity securities, due after five years through ten years, amortized cost	3,544.2
Fixed maturity securities, due after ten years, amortized cost	190.4
Fixed maturity securities, due in one year or less, estimated fair value	796.5
Fixed maturity securities, due after one year through five years, estimated fair value	4,145.8
Fixed maturity securities, due after five years through ten years, estimated fair value	3,579.4
Fixed maturity securities, due after ten years, estimated fair value	190.5
Bonds on deposit with Governmental Authorities	551.2
Other than temporary impairment losses, Equity securities	0		131.3
Equity securities, adjusted cost	632.0		452.1
Available-for-sale Securities, Equity Securities	1,004.2		739.7
Fixed maturity securities and equity, less than twelve months, fair value	2,655.0		491.5
Fixed maturity securities and equity, 12 months or less, unrealized losses	70.0		7.8
Fixed maturity securities and equity, greater than twelve months, fair value	47.7		10.2
Fixed maturity securities and equity, greater than twelve months, unrealized losses	4.1		1.7
Fixed maturity securities and equity, total, fair value	2,702.8		501.8
Fixed maturity securities and equity, total, unrealized losses	74.2		9.5
Short-term investments	1,124.8		1,264.9
Gross investment income	323.7		341.6		370.9
Investment expenses	4.9	[1]	5.1	[1]	6.2	[1]
Net investment income	318.7		336.5		364.6
Net realized gains (losses)	96.2		31.1		78.0
Gross realized investment gains (losses)	148.1		47.8		115.5
Income taxes (credits)	51.8	[2]	16.7	[2]	37.5	[2]
Net changes in unrealized investment gains (losses)	(166.2)		73.8		7.5
Interest expense incurred on funds held	1.7		2.0		1.9
Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	298.7		321.0		353.2
Equity Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	21.2		13.1		11.3
Short-term Investments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	1.1		1.9		1.5
Investments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	2.6		5.4		4.7
Fixed Maturity Securities, U S Canadian Governments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed maturity securities, amortized cost	1,133.0		1,151.2
Fixed maturity securities, gross unrealized gains	36.7		65.9
Fixed maturity securities, gross unrealized losses	8.7		0.3
Fixed maturity securities, estimated fair value	1,161.1		1,216.8
Fixed maturity securities and equity, less than twelve months, fair value	301.7		60.3
Fixed maturity securities and equity, 12 months or less, unrealized losses	8.7		0.3
Fixed maturity securities and equity, greater than twelve months, fair value	0		0
Fixed maturity securities and equity, greater than twelve months, unrealized losses	0		0
Fixed maturity securities and equity, total, fair value	301.7		60.3
Fixed maturity securities and equity, total, unrealized losses	8.7		0.3
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed maturity securities, amortized cost	168.1		380.8
Fixed maturity securities, gross unrealized gains	3.7		11.4
Fixed maturity securities, gross unrealized losses	0.5		0.1
Fixed maturity securities, estimated fair value	171.3		392.2
Fixed maturity securities and equity, less than twelve months, fair value	10.0		3.7
Fixed maturity securities and equity, 12 months or less, unrealized losses	0.5		0.1
Fixed maturity securities and equity, greater than twelve months, fair value	0		0
Fixed maturity securities and equity, greater than twelve months, unrealized losses	0		0
Fixed maturity securities and equity, total, fair value	10.0		3.7
Fixed maturity securities and equity, total, unrealized losses	0.5		0.1
Corporate Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed maturity securities, amortized cost	7,176.0		6,461.0
Fixed maturity securities, gross unrealized gains	268.1		502.1
Fixed maturity securities, gross unrealized losses	64.3		6.0
Fixed maturity securities, estimated fair value	7,379.8		6,957.1
Fixed maturity securities and equity, less than twelve months, fair value	2,312.2		348.4
Fixed maturity securities and equity, 12 months or less, unrealized losses	60.2		4.3
Fixed maturity securities and equity, greater than twelve months, fair value	47.7		10.2
Fixed maturity securities and equity, greater than twelve months, unrealized losses	4.1		1.7
Fixed maturity securities and equity, total, fair value	2,360.0		358.6
Fixed maturity securities and equity, total, unrealized losses	64.3		6.0
Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed maturity securities, amortized cost	8,477.3		7,993.1
Fixed maturity securities, gross unrealized gains	308.7		579.5
Fixed maturity securities, gross unrealized losses	73.6		6.5
Fixed maturity securities, estimated fair value	8,712.3		8,566.2
Fixed maturity securities and equity, less than twelve months, fair value	2,624.0		412.6
Fixed maturity securities and equity, 12 months or less, unrealized losses	69.4		4.8
Fixed maturity securities and equity, greater than twelve months, fair value	47.7		10.2
Fixed maturity securities and equity, greater than twelve months, unrealized losses	4.1		1.7
Fixed maturity securities and equity, total, fair value	2,671.8		422.8
Fixed maturity securities and equity, total, unrealized losses	73.6		6.5
Securities in unrealized loss positions, number of positions	558		102
Percentage of securities in unrealized loss position	30.80%		5.70%
Number of securities in continuous unrealized loss position for more than 12 months	10		4
Equity Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Equity securities, adjusted cost	632.0		452.1
Equity securities, gross unrealized gains	372.7		290.5
Equity securities, gross unrealized losses	0.5		2.9
Available-for-sale Securities, Equity Securities	1,004.2		739.7
Fixed maturity securities and equity, less than twelve months, fair value	31.0		78.9
Fixed maturity securities and equity, 12 months or less, unrealized losses	0.5		2.9
Fixed maturity securities and equity, greater than twelve months, fair value	0		0
Fixed maturity securities and equity, greater than twelve months, unrealized losses	0		0
Fixed maturity securities and equity, total, fair value	31.0		78.9
Fixed maturity securities and equity, total, unrealized losses	0.5		2.9
Securities in unrealized loss positions, number of positions	5		14
Percentage of securities in unrealized loss position	7.20%		21.90%
Number of securities in continuous unrealized loss position for more than 12 months	0		1
Fair Value, Inputs, Level 1 [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Short-term investments	1,120.5		1,260.2
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, estimated fair value	478.9		570.9
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, estimated fair value	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, estimated fair value	0		0
Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Available-for-sale Securities, Equity Securities	1,003.4		736.9
Fair Value, Inputs, Level 2 [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Short-term investments	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, estimated fair value	682.2		645.9
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, estimated fair value	171.3		392.2
Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed maturity securities, estimated fair value	7,369.3		6,926.3
Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Available-for-sale Securities, Equity Securities	0		0
Fair Value, Inputs, Level 3 [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Short-term investments	4.2		4.6
Level 3 security disposed of	20.2
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, estimated fair value	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, estimated fair value	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, estimated fair value	10.5		30.7
Fair Value, Inputs, Level 3 [Member] | Equity Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Available-for-sale Securities, Equity Securities	0.7		2.7
Estimate of Fair Value, Fair Value Disclosure [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Short-term investments	1,124.8		1,264.9
Estimate of Fair Value, Fair Value Disclosure [Member] | Fixed Maturity Securities, U S Canadian Governments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed maturity securities, estimated fair value	1,161.1		1,216.8
Estimate of Fair Value, Fair Value Disclosure [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed maturity securities, estimated fair value	171.3		392.2
Estimate of Fair Value, Fair Value Disclosure [Member] | Corporate Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed maturity securities, estimated fair value	7,379.8		6,957.1
Estimate of Fair Value, Fair Value Disclosure [Member] | Equity Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Available-for-sale Securities, Equity Securities	1,004.2		739.7
Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Realized gains on fixed maturity securities	9.9		32.7		143.9
Realized losses on fixed maturity securities	(8.2)		0		(1.2)
Net realized gains (losses)	1.7		32.7		142.6
Changes in unrealized investment gains (losses) on:	(337.9)		64.9		46.3
Less: Deferred income taxes (credits)	(117.9)		22.6		16.0
Net changes in unrealized investment gains (losses)	(219.9)		42.2		30.3
Equity securities & other long-term investments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross realized investment gains (losses)	146.3		15.1		(27.0)
Changes in unrealized investment gains (losses) on:	82.6		48.4		(35.2)
Less: Deferred income taxes (credits)	28.9		16.8		(12.4)
Net changes in unrealized investment gains (losses)	$ 53.7		$ 31.5		$ (22.7)

[1] (a)Investment expenses consist of personnel costs and investment management and custody service fees, as well as interest incurred on funds held of $1.7, $2.0 and $1.9 for the years ended December 31, 2013, 2012 and 2011, respectively.

[2] Reflects primarily the combination of fully taxable realized investment gains or losses and judgments about the recoverability of deferred tax assets.