Segment Information Operating Results (Parenthetical) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Amortization of intangible assets	$ 66.2	$ 62.1	$ 46.8
Net Premiums Written	$ (27,132.1)	$ (23,353.5)	$ (20,564.0)
Personal Lines Segment \| Underwriting Operations \| Operating Segments
Segment Reporting Information [Line Items]
Personal auto insurance percentage of the total personal lines segment net premiums earned	93.00%	92.00%	92.00%
Property Segment \| Underwriting Operations \| Operating Segments
Segment Reporting Information [Line Items]
Amortization of intangible assets	$ 66.2	$ 62.1	$ 46.8
Other Indemnity
Segment Reporting Information [Line Items]
Net Premiums Written	$ 0.0	$ 0.0	$ 0.4

Percentage of segment revenue attributable to personal auto insurance.

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- Definition

The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16225-109274

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Amount of actual written premiums, including, as applicable, premiums from reinsurance assumed and net of premiums on reinsurance ceded.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 235
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-16)
-URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574960-122915

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