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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 |  |  |  |  |  |  |  |

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