Basis of Presentation and Significant Accounting Policies - Basis of Presentation (Details) - USD ($)		3 Months Ended
	Apr. 04, 2019	Dec. 31, 2019	Sep. 30, 2020	Apr. 07, 2020	Mar. 31, 2020	Apr. 03, 2019	Jan. 01, 2019	Feb. 05, 2018	Dec. 31, 2017	Jul. 31, 2012
Basis of Presentation
Payments to acquire business, net	$ 81,400,000
Total commitment under revolving credit facility				$ 291,400,000	$ 234,000,000.0
ROU lease asset		$ 22,600,000	$ 38,300,000				$ 23,200,000
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]			us-gaap:OtherAssets
Operating Lease, Liability, Statement of Financial Position [Extensible List]			us-gaap:OtherLiabilities
Operating lease liability		$ 22,800,000	$ 39,100,000				$ 23,200,000
HG Global
Basis of Presentation
Percentage of par value of policy reinsured			15.00%
Surplus notes										$ 503,000,000.0
Preferred Stock | HG Global
Basis of Presentation
Ownership interest (as a percent)		96.60%	96.90%
Common equity securities | HG Global
Basis of Presentation
Ownership interest (as a percent)		89.60%	88.40%
NSM Insurance HoldCo, LLC
Basis of Presentation
Ownership interest (as a percent)		96.40%	96.60%
Fully diluted ownership interest (as a percent)		88.40%	89.60%
Asset Management (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%
Unfunded capital commitment	$ 250,000,000.0	$ 129,000,000.0	$ 94,500,000			$ 250,000,000.0
Increase (decrease) in capital commitment		100,000,000.0
Funded capital commitment		350,000,000.0
Series A BAM Surplus Note | HG Global
Basis of Presentation
Surplus notes									$ 203,000,000.0	203,000,000.0
Series B BAM Surplus Note | HG Global
Basis of Presentation
Surplus notes										$ 300,000,000.0
Kudu Debt | Delayed Draw Term Loans | Kudu
Basis of Presentation
Total commitment under revolving credit facility		$ 125,000,000.0