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Goodwill and Other Intangible Assets - Schedule of Goodwill and Other Intangible Assets (Details) - USD ($) $ in Millions | Mar. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | --- | ---
Goodwill [Line Items] |  |  |
Goodwill, acquisition date fair value | $ 349.7 | $ 379.9 |
Goodwill | 349.7 | 379.9 | $ 25.9
Other intangible assets, accumulated amortization | 13.8 | 53.6 |
Goodwill and intangible assets, acquisition date fair value | 524.0 | 591.1 |
Goodwill and intangible assets, net carrying value | 510.2 | 537.5 |
NSM Holding Co LLC |  |  |
Goodwill [Line Items] |  |  |
Goodwill, acquisition date fair value | 328.8 | 354.3 |
Goodwill | 328.8 | 354.3 |
Other intangible assets, acquisition date fair value | 173.2 | 140.2 |
Other intangible assets, accumulated amortization | 13.3 | 8.3 |
Other intangible assets, net carrying value | 159.9 | 131.9 |
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 | 20.9 | 7.3 |
Goodwill | 20.9 | 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 | 110.9 | 85.3 |
Other intangible assets, accumulated amortization | 9.3 | 6.0 |
Other intangible assets, net carrying value | 101.6 | 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 | 2.7 | 1.8 |
Other intangible assets, net carrying value | 54.4 | 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 | 3.8 | 3.7 |
Other intangible assets, accumulated amortization | 0.7 | 0.5 |
Other intangible assets, net carrying value | 3.1 | 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 | 1.4 |  |
Other intangible assets, accumulated amortization | 0.6 |  |
Other intangible assets, net carrying value | 0.8 |  |
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 |
Other Intangible Assets |  |  |
Goodwill [Line Items] |  |  |
Other intangible assets, acquisition date fair value | 174.3 | 211.2 |
Other intangible assets, accumulated amortization | 13.8 | 53.6 |
Other intangible assets, net carrying value | 160.5 | 157.6 |
Non-controlling interest |  |  |
Goodwill [Line Items] |  |  |
Goodwill and intangible assets, net carrying value | (24.1) | (40.6) |
Total |  |  |
Goodwill [Line Items] |  |  |
Goodwill and intangible assets, net carrying value | $ 486.1 | $ 496.9 |

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