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