Acquisition Activity - Narrative (Details) $ in Thousands					3 Months Ended	12 Months Ended
	Jul. 07, 2017 USD ($)	Sep. 01, 2016 USD ($) image image_collection	Jan. 22, 2015 USD ($)	Jan. 19, 2015 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Business Acquisition [Line Items]
Cash payment						$ 49,571	$ 0	$ 62,379
FlashStock Technology, Inc.
Business Acquisition [Line Items]
Purchase price	$ 51,700
Cash payment						50,900
Estimated useful life	7 years
Portion of goodwill deductible for income tax purposes	26.00%
Professional fees						$ 800
The Picture Desk
Business Acquisition [Line Items]
Cash payment		$ 3,900
Minimum number of images | image		700,000
Number of image collections | image_collection		2
Rex Features
Business Acquisition [Line Items]
Cash payment				$ 32,700
PremiumBeat
Business Acquisition [Line Items]
Purchase price			$ 35,400
Cash payment			31,700
Contingent consideration			$ 3,700
Contingent consideration settlement amount							$ 10,000
Trade name | The Picture Desk
Business Acquisition [Line Items]
Estimated useful life		7 years
Contributor content		$ 3,600
Images | The Picture Desk
Business Acquisition [Line Items]
Estimated useful life		15 years
Forecast | FlashStock Technology, Inc.
Business Acquisition [Line Items]
Cash payment					$ 800