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Goodwill and Other Intangible Assets - Schedule of Goodwill and Other Intangible Assets (Details) - USD ($) $ in Millions | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2018 | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | --- | --- | --- | ---
Goodwill [Line Items] |  |  |  |  |
Goodwill, acquisition date fair value | $ 428.6 |  | $ 379.9 |  |
Goodwill | 428.6 | $ 349.7 | 379.9 | $ 25.9 | $ 25.9
Other intangible assets, accumulated amortization | 17.9 |  | 53.6 |  |
Goodwill and intangible assets, acquisition date fair value | 685.5 |  | 591.1 |  |
Goodwill and intangible assets, net carrying value | 667.6 |  | 537.5 |  |
NSM Holding Co LLC |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Goodwill, acquisition date fair value | 398.4 |  | 354.3 |  |
Goodwill | 398.4 |  | 354.3 |  |
Other intangible assets, acquisition date fair value | 255.8 |  | 140.2 |  |
Other intangible assets, accumulated amortization | 17.4 |  | 8.3 |  |
Other intangible assets, net carrying value | 238.4 |  | 131.9 |  |
Kudu |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Goodwill, acquisition date fair value | 9.8 |  | 0.0 |  |
Goodwill | 9.8 |  | 0.0 |  |
MediaAlpha |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Goodwill, acquisition date fair value | 0.0 |  | 18.3 |  |
Goodwill | 0.0 |  | 18.3 |  |
Other intangible assets, acquisition date fair value | 0.0 |  | 69.9 |  |
Other intangible assets, accumulated amortization | 0.0 |  | 44.8 |  |
Other intangible assets, net carrying value | 0.0 |  | 25.1 |  |
Other Operations |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Goodwill, acquisition date fair value | 20.4 |  | 7.3 |  |
Goodwill | 20.4 |  | 7.3 |  |
Other intangible assets, acquisition date fair value | 1.1 |  | 1.1 |  |
Other intangible assets, accumulated amortization | 0.5 |  | 0.5 |  |
Other intangible assets, net carrying value | 0.6 |  | 0.6 |  |
Customer relationships | NSM Holding Co LLC |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Other intangible assets, acquisition date fair value | 110.7 |  | 85.3 |  |
Other intangible assets, accumulated amortization | 12.4 |  | 6.0 |  |
Other intangible assets, net carrying value | 98.3 |  | 79.3 |  |
Customer relationships | MediaAlpha |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Other intangible assets, acquisition date fair value | 0.0 |  | 26.8 |  |
Other intangible assets, accumulated amortization | 0.0 |  | 4.9 |  |
Other intangible assets, net carrying value | 0.0 |  | 21.9 |  |
Trade names | NSM Holding Co LLC |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Other intangible assets, acquisition date fair value | 57.1 |  | 51.2 |  |
Other intangible assets, accumulated amortization | 3.5 |  | 1.8 |  |
Other intangible assets, net carrying value | 53.6 |  | 49.4 |  |
Trade names | Other Operations |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Other intangible assets, acquisition date fair value | 0.6 |  | 0.6 |  |
Other intangible assets, accumulated amortization | 0.2 |  | 0.2 |  |
Other intangible assets, net carrying value | 0.4 |  | 0.4 |  |
Information technology | NSM Holding Co LLC |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Other intangible assets, acquisition date fair value | 3.7 |  | 3.7 |  |
Other intangible assets, accumulated amortization | 0.9 |  | 0.5 |  |
Other intangible assets, net carrying value | 2.8 |  | 3.2 |  |
Information technology | MediaAlpha |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Other intangible assets, acquisition date fair value | 0.0 |  | 33.3 |  |
Other intangible assets, accumulated amortization | 0.0 |  | 30.9 |  |
Other intangible assets, net carrying value | 0.0 |  | 2.4 |  |
Information technology | Other Operations |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Other intangible assets, acquisition date fair value | 0.5 |  | 0.5 |  |
Other intangible assets, accumulated amortization | 0.3 |  | 0.3 |  |
Other intangible assets, net carrying value | 0.2 |  | 0.2 |  |
Other Operations | NSM Holding Co LLC |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Other intangible assets, acquisition date fair value | 1.8 |  |  |  |
Other intangible assets, accumulated amortization | 0.6 |  |  |  |
Other intangible assets, net carrying value | 1.2 |  |  |  |
Other Operations | MediaAlpha |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Other intangible assets, acquisition date fair value | 0.0 |  | 9.8 |  |
Other intangible assets, accumulated amortization | 0.0 |  | 9.0 |  |
Other intangible assets, net carrying value | 0.0 |  | 0.8 |  |
Renewal Rights | NSM Holding Co LLC |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Other intangible assets, acquisition date fair value | 82.5 |  |  |  |
Other intangible assets, accumulated amortization | 0.0 |  |  |  |
Other intangible assets, net carrying value | 82.5 |  |  |  |
Other Intangible Assets |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Other intangible assets, acquisition date fair value | 256.9 |  | 211.2 |  |
Other intangible assets, accumulated amortization | 17.9 |  | 53.6 |  |
Other intangible assets, net carrying value | 239.0 |  | 157.6 |  |
Non-controlling interest |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Goodwill and intangible assets, net carrying value | (25.4) |  | (40.6) |  |
Total |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Goodwill and intangible assets, net carrying value | $ 642.2 |  | $ 496.9 |  |

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