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Goodwill and Other Intangible Assets - Narrative (Details) - USD ($) |  |  |  | 6 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jun. 28, 2019 | Dec. 03, 2018 | May 11, 2018 | Jun. 30, 2019 | Jun. 30, 2018 | Apr. 04, 2019 | Apr. 01, 2019 | Feb. 26, 2019 | Dec. 31, 2018 | May 18, 2018
Goodwill [Line Items] |  |  |  |  |  |  |  |  |  |
Purchases of consolidated subsidiaries, net of cash acquired |  |  |  | $ 243,400,000 | $ 252,100,000 |  |  |  |  |
NSM Insurance HoldCo, LLC |  |  |  |  |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |  |  |  |  |
Goodwill and intangible assets acquired |  |  | $ 383,000,000.0 |  |  |  |  |  |  |
Purchases of consolidated subsidiaries, net of cash acquired |  |  | $ 274,200,000 |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  | 95.00% | 96.40% |  |  |  |  | 95.50% |
Fully diluted ownership interest (as a percent) |  |  |  | 88.40% |  |  |  |  | 85.00% |
Fresh Insurance Services Group Limited |  |  |  |  |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |  |  |  |  |
Goodwill and intangible assets acquired |  |  |  |  |  |  |  |  |  | $ 54,600,000
Ownership interest (as a percent) |  |  |  |  |  |  |  |  |  | 100.00%
KBK Insurance Group, Inc. & KBK Premium Services, Inc. |  |  |  |  |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |  |  |  |  |
Goodwill and intangible assets acquired |  | $ 59,400,000 |  | $ 32,600,000 |  |  |  |  |  |
Intangible assets acquired |  |  |  | $ 32,700,000 |  |  |  |  |  |
Purchases of consolidated subsidiaries, net of cash acquired |  | $ 60,000,000.0 |  |  |  |  |  |  |  |
AIG Collector Car Business |  |  |  |  |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |  |  |  |  |
Intangible assets acquired | $ 82,500,000 |  |  |  |  |  |  |  |  |
Purchases of consolidated subsidiaries, net of cash acquired | $ 82,500,000 |  |  |  |  |  |  |  |  |
Acquired finite-lived intangible assets, weighted average useful life (in years) | 12 years |  |  |  |  |  |  |  |  |
MediaAlpha |  |  |  |  |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  |  |  |  |  |  | 48.30% |  |
Fully diluted ownership interest (as a percent) |  |  |  |  |  |  |  | 42.00% |  |
Embrace Pet Insurance | NSM Holding Co LLC |  |  |  |  |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |  |  |  |  |
Goodwill and intangible assets acquired |  |  |  |  |  |  | $ 70,600,000 |  |  |
Kudu | White Mountains |  |  |  |  |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |  |  |  |  |
Goodwill and intangible assets acquired |  |  |  |  |  | $ 9,800,000 |  |  |  |

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