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Investment Securities (Other Long-Term Investments) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Investment securities
Other Investments	$ 622.8	$ 165.7
Kudu investments
Investment securities
Other Investments	266.5	0.0
MediaAlpha
Investment securities
Other Investments	180.0	0.0
DavidShield & PassportCard
Investment securities
Other Investments	90.0	75.0
Elementum Holdings, L.P.
Investment securities
Other Investments	55.1	0.0
Other unconsolidated private equity investments, at fair value
Investment securities
Other Investments	31.2	60.0
Kudu
Investment securities
Other Investments	0.0	30.7
Hedge Funds and Private Equity Funds
Investment securities
Other Investments	161.1	146.1
Insurance-linked securities funds
Investment securities
Other Investments	41.2	0.0
Other long-term investments
Investment securities
Other Investments	31.2	13.8
White Mountains
Investment securities
Other Investments	$ 856.3	$ 325.6

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- 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=119400593&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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