Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended									12 Months Ended
	Oct. 02, 2017	Dec. 31, 2017	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Selected Quarterly Financial Information [Abstract]
Revenue		$ 427.1		$ 289.2	$ 294.0	$ 278.2	$ 285.3	$ 312.4	$ 299.2	$ 288.9	$ 1,288.5	$ 1,185.8	$ 1,167.8
Operating (loss) income		9.8		12.7	20.9	(5.3)	10.5	25.5	27.4	29.6	38.1	93.0	128.0
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent							(1.6)	18.8	77.6	7.0	35.1	101.8	290.8
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent							0.3	0.0	0.0	0.0	0.0	0.3	62.9
Net income (loss)		$ (16.2)		$ (11.2)	$ 2.1	$ 60.4	$ (1.3)	$ 18.8	$ 77.6	$ 7.0	$ 35.1	$ 102.1	$ 353.7
Income (Loss) from Continuing Operations, Per Basic Share							$ (0.10)	$ 0.39	$ 1.79	$ 0.10		$ 2.17
Income (Loss) from Discontinued Operations and Disposal of Discontinued Operations, Net of Tax, Per Basic Share							0.01	0.00	0.00	0.00		0.01
Earnings Per Share, Basic		$ (0.45)		$ (0.33)	$ (0.01)	$ 1.37	(0.09)	0.39	1.79	0.10	$ 0.59	2.18	$ 8.19
Income (Loss) from Continuing Operations, Per Diluted Share							(0.10)	0.38	1.78	0.10		2.17
Income from Discontinued Operations and Disposal of Discontinued Operations, Net of Tax, Per Diluted Share							0.01	0.00	0.00	0.00		0.01
Earnings Per Share, Diluted		$ (0.45)		$ (0.33)	$ (0.01)	$ 1.37	$ (0.09)	$ 0.38	$ 1.78	$ 0.10	$ 0.58	$ 2.18	$ 8.17
Restructuring and severance related charges		$ 3.5			$ 3.6	$ 25.6	$ 11.9				$ 32.7	$ 11.9	$ 6.0
Transaction and integration cost		4.1		$ 12.1	$ 1.7	0.6					18.5	0.0	1.4
Marketable Securities, Realized Gain (Loss)						$ 117.7	5.1	$ 33.3	$ 118.6		117.7	157.0	449.2
Gains (Losses) on Extinguishment of Debt		(3.2)					$ (4.8)	$ (11.4)	$ (5.2)	$ (2.4)	$ 3.2	$ 19.0	$ 20.9
Condensed Income Statements, Captions [Line Items]
Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Change In Tax Rate, Deferred Tax Asset, Provisional Income Tax Expense		$ 6.8
OnX Holdings LLC [Member]
Selected Quarterly Financial Information [Abstract]
Transaction and integration cost	$ 8.1
Condensed Income Statements, Captions [Line Items]
Revenue of acquiree since acquisition date included in consolidated statement of income			$ 150.0
Net income of acquiree since acquisition date included in consolidated statement of income			$ 11.5