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Significant Transactions Significant Transactions (Acquisitions) (Details) - USD ($) $ in Millions |  |  |  |  | 12 Months Ended
--- | --- | --- | --- | --- | ---
Oct. 05, 2017 | Aug. 01, 2017 | Aug. 04, 2016 | Jan. 15, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |
Assets |  |  |  |  | $ 3,659.2 | $ 6,520.2 |
Goodwill |  |  |  |  | 25.9 | 25.9 | $ 18.6
Intangible assets, net (excluding goodwill) |  |  |  |  | 36.2 | 19.3 |
Liabilities |  |  |  |  | 298.4 | 2,804.2 |
Payments to acquire businesses and interest in affiliates |  |  |  |  | $ 27.6 | 13.4 | $ 2.6
Buzzmove |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  | 70.90% |  | 77.10% |  |
Assets |  |  | $ 11.5 |  |  |  |
Goodwill |  |  | 7.6 |  | $ 7.6 | 7.6 |
Intangible assets, net (excluding goodwill) |  |  | 1.1 |  | $ 0.8 | $ 1.0 |
Liabilities |  |  | 0.1 |  |  |  |
LowFares.com |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |
Payments to acquire businesses and interest in affiliates |  |  |  | $ 3.9 |  |  |
Healthplans.com |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |
Payments to acquire businesses and interest in affiliates | $ 28.0 |  |  |  |  |  |
MediaAlpha |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  |  |  | 64.40% | 60.00% |
Assets |  |  |  |  | $ 96.5 | $ 57.6 |
Liabilities |  |  |  |  | $ 59.8 | $ 28.3 |
Preferred stocks | Buzzmove |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |
Payments to Acquire Interest in Subsidiaries and Affiliates |  | $ 5.0 |  |  |  |  |
Stock issued during period, shares, acquisitions |  | 37,409 |  |  |  |  |
Common Stock | Buzzmove |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |
Payments to Acquire Interest in Subsidiaries and Affiliates |  | $ 0.7 |  |  |  |  |
Stock issued during period, shares, acquisitions |  | 5,808 |  |  |  |  |
GBP | Buzzmove |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |
Payments to Acquire Interest in Subsidiaries and Affiliates |  |  | 6.1 |  |  |  |
GBP | Preferred stocks | Buzzmove |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |
Payments to Acquire Interest in Subsidiaries and Affiliates |  | $ 4.0 |  |  |  |  |
GBP | Common Stock | Buzzmove |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |
Payments to Acquire Interest in Subsidiaries and Affiliates |  | $ 0.5 |  |  |  |  |
United States of America, Dollars | Buzzmove |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |
Payments to Acquire Interest in Subsidiaries and Affiliates |  |  | $ 8.1 |  |  |  |
Class A unit | MediaAlpha |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |
Capital units, purchased | 131,579 |  |  |  |  |  |
Capital units, value | $ 12.5 |  |  |  |  |  |

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