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Basis of Presentation and Significant Accounting Policies (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended
	Jan. 01, 2021	Mar. 31, 2021	Mar. 23, 2021	Dec. 31, 2020	Oct. 30, 2020	Mar. 31, 2020	Dec. 31, 2017	Dec. 31, 2012	Jul. 31, 2012
Marketing Technology (MediaAlpha)
Basis of Presentation
Ownership interest (as a percent)			28.50%		35.00%
Share price (in USD per share)		$ 35.43		$ 39.07	$ 19.00
Ark
Basis of Presentation
Investment, potential additional contribution of equity capital	$ 200.0	$ 200.0
Fully diluted ownership interest after additional funding		67.50%
Ownership interest after additional funding		77.10%
Percent of shares outstanding		12.50%
Ark
Basis of Presentation
Percent of underwriting capital provided by third parties						51.00%
NSM Insurance HoldCo, LLC
Basis of Presentation
Ownership interest (as a percent)		96.60%
Fully diluted ownership interest (as a percent)		89.60%
Asset Management (Kudu)
Basis of Presentation
Ownership interest (as a percent)		99.30%
Fully diluted ownership interest (as a percent)		84.80%
HG Global
Basis of Presentation
Percentage of par value of policy reinsured		15.00%
Surplus notes								$ 503.0	$ 503.0
HG Global | Preferred Stock
Basis of Presentation
Ownership interest (as a percent)		96.90%		96.90%
HG Global | Common equity securities
Basis of Presentation
Ownership interest (as a percent)		88.40%		88.40%
HG Global | Series A BAM Surplus Note
Basis of Presentation
Surplus notes							$ 203.0		203.0
HG Global | Series B BAM Surplus Note
Basis of Presentation
Surplus notes									$ 300.0
Ark
Basis of Presentation
Ownership interest (as a percent)		72.00%
Ark | Ark
Basis of Presentation
Ownership interest (as a percent)		72.00%
Fully diluted ownership interest (as a percent)		63.00%
Investment, potential additional contribution of equity capital	$ 200.0
Fully diluted ownership interest after additional funding	67.50%
Ownership interest after additional funding	77.10%
Ark | Ark
Basis of Presentation
Fully diluted ownership interest (as a percent)	63.00%
Ark | Third Point Re
Basis of Presentation
Reinsurance recoverable, aggregate excess loss		$ 50.0
Annual crediting rate		3.00%
Carrying value of investment		$ 51.7

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