Investments - Proceeds From Sales of Available-For-Sale Securities and the Gross Realized Gains and Gross Realized Losses (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Available-for-sale securities, proceeds and realized gains (losses) included in earnings
Proceeds from sales	$ 218.7	$ 1,939.4	$ 312.0
Gross realized gains	7.1	168.9	9.7
Gross realized losses	$ (0.4)	(19.9)	$ (4.3)
Other information
Average period of time for which securities were traded continuously at a price below book value for securities sold at a loss	2 months
Disposal Group, Disposed of by Sale, Not Discontinued Operations \| Assurant Employee Benefits
Available-for-sale securities, proceeds and realized gains (losses) included in earnings
Gross realized gains		145.6
Gross realized losses		$ (15.9)

Represents the average period of time for which securities were traded continuously at a price below book value.

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- References

No definition available.

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- Definition

This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563

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- Definition

This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

+ References

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- References

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- Definition

The cash inflow associated with the sale or maturity (principal being due) of securities not classified as either held-to-maturity securities or trading securities which are classified as available-for-sale securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (a),(b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3179-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26853-111562

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