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Significant Transactions (Acquisitions) (Details) |  |  |  |  |  |  | 3 Months Ended | 8 Months Ended | 12 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jun. 28, 2019 USD ($) | Apr. 04, 2019 USD ($) | Apr. 01, 2019 USD ($) | Dec. 03, 2018 USD ($) | May 11, 2018 USD ($) | Jan. 24, 2018 USD ($) | Dec. 31, 2019 USD ($) | Mar. 31, 2019 USD ($) | Sep. 30, 2018 USD ($) | Jun. 30, 2018 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2019 USD ($) earnout_period | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | May 31, 2019 USD ($) | Apr. 03, 2019 | Feb. 26, 2019 | May 18, 2018 USD ($) | Feb. 05, 2018 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Investments |  |  |  |  |  |  | $ 622,800,000 |  |  |  | $ 165,700,000 | $ 622,800,000 | $ 165,700,000 |  |  |  |  |  |
Equity method investment, aggregate cost |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 125,000,000.0
Purchases of consolidated subsidiaries, net of cash acquired |  |  |  |  |  |  |  |  |  |  |  | 258,000,000.0 | 295,200,000 | $ 27,600,000 |  |  |  |  |
Change in fair value of contingent consideration earnout liabilities |  |  |  |  |  |  |  |  |  |  |  | 2,100,000 | 2,700,000 |  |  |  |  |  |
Payments to acquire business, net |  | $ 81,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Goodwill |  |  |  |  |  |  | 394,700,000 |  |  |  | 379,900,000 | 394,700,000 | 379,900,000 |  |  |  |  |  |
NSM Holding Co LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term line of credit |  |  |  |  | $ 100,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Change in fair value of contingent consideration earnout liabilities |  |  |  |  |  |  |  |  |  |  | 2,700,000 | 2,100,000 |  |  |  |  |  |  |
Contingent consideration earnout liabilities |  |  |  |  |  |  | 20,600,000 |  |  |  | 20,200,000 | 20,600,000 | 20,200,000 |  |  |  |  |  |
White Mountains |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Investments |  |  |  |  |  |  | $ 856,300,000 |  |  |  | $ 325,600,000 | $ 856,300,000 | $ 325,600,000 |  |  |  |  |  |
MediaAlpha |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 48.30% |  |
Fully diluted ownership interest (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 42.00% |  |
Elementum Holdings, L.P. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  |  |  |  |  | 30.00% |  |  |  |  | 30.00% |  |  | 30.00% |  |  |  |
Other Investments |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 55,100,000 |  |  |  |
Insurance-Linked Securities Funds |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commitment |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 50,000,000.0 |  |  |  |
Elementum Funds |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment, aggregate cost |  |  |  |  |  |  | $ 40,000,000.0 |  |  |  |  | $ 40,000,000.0 |  |  |  |  |  |  |
NSM Insurance HoldCo, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  |  |  | 95.00% |  | 96.40% |  |  |  | 95.50% | 96.40% | 95.50% |  |  |  |  |  |
Purchases of consolidated subsidiaries, net of cash acquired |  |  |  |  | $ 274,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase price adjustment |  |  |  |  |  |  |  |  | $ 2,100,000 |  |  |  |  |  |  |  |  |  |
Assets acquired |  |  |  |  | 495,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Goodwill and intangible assets acquired |  |  |  |  | 383,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Liabilities acquired |  |  |  |  | 204,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Contingent consideration |  |  |  |  | 10,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Noncontrolling interest acquired |  |  |  |  | $ 14,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Transaction costs |  |  |  |  |  |  |  |  |  | $ 6,300,000 |  |  |  |  |  |  |  |  |
Number of earnout periods | earnout_period |  |  |  |  |  |  |  |  |  |  |  | 2 |  |  |  |  |  |  |
Fully diluted ownership interest (as a percent) |  |  |  |  | 83.60% |  | 88.40% |  |  |  | 85.00% | 88.40% | 85.00% |  |  |  |  |  |
Fresh Insurance Services Group Limited |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100.00% |
Equity method investment, aggregate cost |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 49,600,000 |
Purchase price adjustment |  |  |  |  |  |  |  | $ 700,000 |  |  |  |  |  |  |  |  |  |  |
Assets acquired |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 72,600,000 |
Goodwill and intangible assets acquired |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 54,600,000 |
Liabilities acquired |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 22,300,000 |
Contingent consideration |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7,500,000 |
KBK Insurance Group, Inc. & KBK Premium Services, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchases of consolidated subsidiaries, net of cash acquired |  |  |  | $ 60,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Goodwill and intangible assets acquired |  |  |  | 59,400,000 |  |  | $ 32,600,000 |  |  |  |  | $ 32,600,000 |  |  |  |  |  |  |
Stock issued during period, value, acquisitions |  |  |  | 29,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from borrowings |  |  |  | $ 30,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangible assets acquired |  |  |  |  |  |  | $ 32,700,000 |  |  |  |  | $ 32,700,000 |  |  |  |  |  |  |
Change in fair value of contingent consideration earnout liabilities |  |  |  |  |  |  |  | $ 5,900,000 |  |  |  |  |  |  |  |  |  |  |
AIG Collector Car Business |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchases of consolidated subsidiaries, net of cash acquired | $ 82,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangible assets acquired | 82,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
AIG Collector Car Business | NSM Holding Co LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from borrowings | 22,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
AIG Collector Car Business | White Mountains |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock issued during period, value, acquisitions | $ 59,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
DavidShield |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |
PassportCard |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |
DavidShield & PassportCard |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  |  |  |  |  | 50.00% |  |  |  | 50.00% | 50.00% | 50.00% |  |  |  |  |  |
Payments to acquire businesses, gross |  |  |  |  |  | $ 41,800,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to acquire business, net |  |  |  |  |  | $ 28,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Kudu |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  | 99.10% |  |  |  |  |  |  |  |  |  |  |  |  |  | 49.50% |  |  | 49.50%
Assets acquired |  | $ 155,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 155,500,000
Liabilities acquired |  | 800,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 800,000
Noncontrolling interest acquired |  | 1,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,500,000
Goodwill |  | 7,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7,600,000
Intangible assets acquired |  | $ 2,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,200,000
Fully diluted ownership interest (as a percent) |  | 85.40% |  |  |  |  |  |  |  |  |  |  |  |  |  | 42.70% |  |  | 42.70%
Funded capital commitment |  | $ 250,000,000.0 |  |  |  |  | $ 350,000,000.0 |  |  |  |  | $ 350,000,000.0 |  |  |  |  |  |  |
Increase in capital commitment |  |  |  |  |  |  | 100,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |
Unfunded capital commitment |  |  |  |  |  |  | 129,000,000.0 |  |  |  |  | 129,000,000.0 |  |  |  |  |  |  |
NSM Bank Facility | NSM Holding Co LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total commitment under revolving credit facility |  |  |  |  | $ 234,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
NSM Bank Facility | Term Loan | NSM Holding Co LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term line of credit |  |  |  |  |  |  | 221,300,000 |  |  |  | $ 180,400,000 | 221,300,000 | $ 180,400,000 |  |  |  |  | $ 51,000,000.0 |
Proceeds from borrowings |  |  |  |  |  |  |  |  |  |  |  | 42,900,000 |  |  |  |  |  |  |
Total commitment under revolving credit facility |  |  |  |  | $ 224,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Kudu Debt | Delayed Draw Term Loans | Kudu |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total commitment under revolving credit facility |  |  |  |  |  |  | 125,000,000.0 |  |  |  |  | 125,000,000.0 |  |  |  |  |  |  |
Undrawn amount on facility |  |  |  |  |  |  | $ 68,000,000.0 |  |  |  |  | 68,000,000.0 |  |  |  |  |  |  |
Embrace Pet Insurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Goodwill and intangible assets acquired |  |  | $ 67,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangible assets acquired |  |  | 15,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Goodwill |  |  | 52,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Embrace Pet Insurance | NSM Holding Co LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock issued during period, value, acquisitions |  |  | 58,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from borrowings |  |  | $ 20,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of voting interests acquired |  |  | 100.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to acquire businesses, gross |  |  | $ 71,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Embrace Pet Insurance | NSM Bank Facility | Term Loan | NSM Holding Co LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from borrowings |  |  |  |  |  |  |  |  |  |  |  | $ 20,400,000 |  |  |  |  |  |  |

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