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Investment Securities (Other long-term investments) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	$ 114.7	$ 0.0
DavidShield & PassportCard
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	75.0	75.0
Kudu Investment Management, LLC
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	71.7	30.7
Other unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	54.4	60.0
Total unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	315.8	165.7
Hedge and private equity funds included in other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	150.7	146.1
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	31.1	13.8
White Mountains
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	$ 497.6	$ 325.6

X

- Definition

Amount of investments classified as other.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(f,g))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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+ References

No definition available.

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