Stock-based Payments - Summary of Deferred Purchase Obligation Compensation Expense (Details) - Cassidy Turley, Inc. - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cassidy Turley deferred payment obligation	$ 5.2	$ 5.5	$ 15.9	$ 16.9
Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cassidy Turley deferred payment obligation	2.7	2.8	7.5	8.7
Non-employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cassidy Turley deferred payment obligation	$ 2.5	$ 2.7	$ 8.4	$ 8.2