Segment Information Operating Results (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Revenues	$ 26,839.0	$ 23,441.4	$ 20,853.8
Pretax Profit (Loss)	2,138.9	1,470.7	1,911.6
Personal Lines Segment \| Agency Channel \| Underwriting Operations
Segment Reporting Information [Line Items]
Revenues	11,177.6	9,791.7	9,108.6
Pretax Profit (Loss)	839.6	492.8	713.2
Personal Lines Segment \| Direct Channel \| Underwriting Operations
Segment Reporting Information [Line Items]
Revenues	10,769.6	9,396.5	8,185.9
Pretax Profit (Loss)	683.7	412.2	403.4
Operating Segments \| Underwriting Operations
Segment Reporting Information [Line Items]
Revenues	25,729.9	22,474.0	19,899.1
Pretax Profit (Loss)	1,686.9	1,091.1	1,495.2
Operating Segments \| Personal Lines Segment \| Underwriting Operations
Segment Reporting Information [Line Items]
Revenues	21,947.2	19,188.2	17,294.5
Pretax Profit (Loss)	1,523.3	905.0	1,116.6
Operating Segments \| Commercial Lines Segment \| Underwriting Operations
Segment Reporting Information [Line Items]
Revenues	2,793.9	2,421.3	1,995.9
Pretax Profit (Loss)	214.1	155.2	318.3
Operating Segments \| Property Segment \| Underwriting Operations
Segment Reporting Information [Line Items]
Revenues	988.8	864.5	609.1
Pretax Profit (Loss)	(50.3)	32.5	61.3
Operating Segments \| Other Indemnity \| Underwriting Operations
Segment Reporting Information [Line Items]
Revenues	0.0	0.0	(0.4)
Pretax Profit (Loss)	(0.2)	(1.6)	(1.0)
Segment Reconciling Items \| Fees And Other Revenues
Segment Reporting Information [Line Items]
Revenues	370.6	332.5	302.0
Segment Reconciling Items \| Service Businesses
Segment Reporting Information [Line Items]
Revenues	126.8	103.3	86.3
Pretax Profit (Loss)	17.3	11.3	8.8
Segment Reconciling Items \| Investments
Segment Reporting Information [Line Items]
Revenues	612.7	530.0	567.3
Pretax Profit (Loss)	588.8	507.6	544.5
Segment Reconciling Items \| Other gains (losses)
Segment Reporting Information [Line Items]
Revenues	(1.0)	1.6	(0.9)
Pretax Profit (Loss)	(1.0)	1.6	(0.9)
Segment Reconciling Items \| Interest Expense
Segment Reporting Information [Line Items]
Pretax Profit (Loss)	$ (153.1)	$ (140.9)	$ (136.0)

Amount of income (loss) from continuing operations before deduction of income tax expense (benefit) and income (loss) attributable to noncontrolling interest, and addition of income (loss) from equity method investments.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(h)(1)(i))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

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