Business Segments - Recast Quarterly Selected Financial Data (Details) - USD ($) $ in Millions	3 Months Ended									6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2018	Jun. 30, 2017
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenue	$ 844.7			$ 766.3						$ 1,656.5	$ 1,491.7
Operating income (loss)	$ 99.5			117.3						$ 148.9	$ 205.3
Social
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenue		$ 95.5	$ 95.1	91.1	$ 80.2	$ 74.8	$ 70.3	$ 69.1	$ 60.2
Operating income (loss)		21.8	20.1	21.9	17.9	15.4	10.9	16.0	13.2
Digital
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenue		17.8	16.3	15.7	16.1	16.8	14.9	14.3	12.4
Operating income (loss)		5.1	3.1	2.7	5.1	4.3	2.7	2.2	2.2
Interactive
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenue		$ 113.3	$ 111.4	$ 106.8	$ 96.3	$ 91.6	$ 85.2	$ 83.4	$ 72.6