Basis of Presentation and Significant Accounting Policies - Reportable Segments (Details) - USD ($) $ in Millions	Oct. 30, 2020	Apr. 04, 2019	Dec. 31, 2020	Dec. 31, 2019	Apr. 03, 2019	May 11, 2018	Feb. 05, 2018	Dec. 31, 2012
Basis of Presentation
Percentage of par value of policy reinsured			15.00%
Payments to acquire business, net		$ 81.4
MediaAlpha
Basis of Presentation
Ownership interest (as a percent)	35.00%
Fully diluted ownership interest (as a percent)	32.30%
Investment owned (in shares)	20,532,202
IPO | MediaAlpha
Basis of Presentation
Sale of stock, number of shares issued in transaction (in shares)	3,609,894
Sale of stock, consideration received in transaction	$ 63.8
HG Global
Basis of Presentation
Percentage of par value of policy reinsured			15.00%
Surplus notes								$ 503.0
Preferred stocks | HG Global
Basis of Presentation
Ownership interest (as a percent)			96.90%	96.90%
Common equity securities | HG Global
Basis of Presentation
Ownership interest (as a percent)			88.40%	88.40%
NSM
Basis of Presentation
Ownership interest (as a percent)			96.60%	96.40%		95.00%
Fully diluted ownership interest (as a percent)			89.60%	88.40%		83.60%
Kudu
Basis of Presentation
Ownership interest (as a percent)		99.10%			49.50%		49.50%
Fully diluted ownership interest (as a percent)		85.40%			42.70%		42.70%
Payments to acquire business, net		$ 81.4
Funded capital commitment		$ 250.0		$ 350.0