Investments in Unconsolidated Entities (Details) - White Mountains - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Investments in unconsolidated affiliates
Other Investments	$ 773.7	$ 325.6
Equity method eligible private equity securities, at fair value
Investments in unconsolidated affiliates
Other Investments	673.2	269.7
Other unconsolidated investments
Investments in unconsolidated affiliates
Other Investments	$ 100.5	$ 55.9

Amount of investments classified as other.

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

Name:	us-gaap_OtherInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant