Business Segments - Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015 segments	Mar. 31, 2014
Segment Reporting Information [Line Items]
Number of Operating Segments	2us-gaap_NumberOfOperatingSegments
Earned Premiums	$ 431.3us-gaap_PremiumsEarnedNet	$ 477.6us-gaap_PremiumsEarnedNet
Net Investment Income	70.6us-gaap_NetInvestmentIncome	71.1us-gaap_NetInvestmentIncome
Other Income	0.9us-gaap_OtherIncome	0.1us-gaap_OtherIncome
Total Revenues	499.2us-gaap_Revenues	554.6us-gaap_Revenues
Net Realized Gains on Sales of Investments	3.4	6.6
Segment Operating Profit (Loss)	30.5us-gaap_OperatingIncomeLoss	46.1us-gaap_OperatingIncomeLoss
Net Impairment Losses Recognized in Earnings	(7.0)	(0.8)
Loss from Early Extinguishment of Debt	9.1	0
Income from Continuing Operations before Income Taxes	17.8	51.9
Segment Net Operating Income (Loss)	21.8kmpr_OperatingIncomeLossNetOfTaxes	31.5kmpr_OperatingIncomeLossNetOfTaxes
Income from Continuing Operations	13.5	35.2
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Segment Operating Profit (Loss)	(12.3)	(7.8)
Segment Net Operating Income (Loss)	(7.7)	(5.0)
Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Other Income	5.2	3.3
Net Realized Gains on Sales of Investments	3.4	6.6
Net Impairment Losses Recognized in Earnings	(7.0)	(0.8)
Loss from Early Extinguishment of Debt	(9.1)	0
Net Realized Gains (Losses) on Sales of Investments, Net of Tax	2.2	4.2
Net Impairment Losses Recognized in Earnings, Net of Tax	(4.6)	(0.5)
Gains (Losses) on Extinguishment of Debt, Net of Taxes	(5.9)	0
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	497.6	545.5
Segment Operating Profit (Loss)	42.8	53.9
Segment Net Operating Income (Loss)	29.5	36.5
Operating Segments [Member] \| Property and Casualty Insurance [Member]
Segment Reporting Information [Line Items]
Earned Premiums	287.6	322.3
Net Investment Income	14.8	17.6
Other Income	0.3	0.1
Total Revenues	302.7	340.0
Segment Operating Profit (Loss)	18.0	19.3
Segment Net Operating Income (Loss)	13.4	14.4
Operating Segments [Member] \| Life and Health Insurance [Member]
Segment Reporting Information [Line Items]
Earned Premiums	143.7	155.3
Net Investment Income	50.4	50.2
Other Income	0.8	0
Total Revenues	194.9	205.5
Segment Operating Profit (Loss)	24.8	34.6
Segment Net Operating Income (Loss)	$ 16.1	$ 22.1

Gain (Loss) on Sale of Investments, Net of Tax

No definition available.

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- Definition

Gains (Losses) on Extinguishment of Debt, Net of Taxes

+ References

No definition available.

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- Definition

Operating Income Loss, Net Of Taxes

+ References

No definition available.

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- Definition

Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net of Tax

+ References

No definition available.

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- Definition

This item represents the net total realized and unrealized gain (loss) included in earnings for the period as a result of selling or holding marketable securities categorized as trading, available-for-sale, or held-to-maturity, including the unrealized holding gain (loss) of held-to-maturity securities transferred to the trading security category and the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) for available-for-sale securities transferred to trading securities during the period. Additionally, this item would include any gains (losses) realized during the period from the sale of investments accounted for under the cost method of accounting.

+ References

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

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-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph ((b)(7))

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- Definition

Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

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-Subparagraph (b)

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-Topic 470

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-Section 40

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-Paragraph 4

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- Definition

Sum of operating profit and nonoperating income or expense before Income or Loss from equity method investments, income taxes, extraordinary items, and noncontrolling interest.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(h)(1)(i))

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- Definition

This element represents the income or loss from continuing operations attributable to the economic entity which may also be defined as revenue less expenses and taxes from ongoing operations before extraordinary items, and noncontrolling interest.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 810

-SubTopic 10

-Section 45

-Paragraph 19

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- Definition

Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 16

-Article 12

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.2)

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 2

-Article 7

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- Definition

Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

+ References

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- Definition

The net result for the period of deducting operating expenses from operating revenues.

+ References

No definition available.

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- Definition

Reflects the sum of all other revenue and income recognized by the entity in the period not otherwise specified in the income statement.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.4)

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-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 4

-Article 7

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- Definition

The amount of other than temporary impairment (OTTI) losses on equity securities, OTTI related to credit losses on debt securities, and OTTI losses on debt securities when the entity intends to sell the securities or it is more likely than not that the entity will be required to sell the securities before recovery of its amortized cost basis. Additionally, this item includes OTTI losses recognized during the period on investments accounted for under the cost method of accounting.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 8B

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- Definition

Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned.

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 17

-Article 12

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-Topic 942

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-Paragraph 1

-Subparagraph (SX 210.9-04.13(c))

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- Definition

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

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