Significant Accounting Policies - Goodwill, Other Intangible and Long-Lived Assets (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Acquired Finite-Lived Intangible Assets [Line Items]
Impairment of Intangible Assets (Excluding Goodwill)	$ 0.0
Customer and advertiser relationships | Minimum
Acquired Finite-Lived Intangible Assets [Line Items]
Estimated useful life of definite-lived intangible asset	5 years
Customer and advertiser relationships | Maximum
Acquired Finite-Lived Intangible Assets [Line Items]
Estimated useful life of definite-lived intangible asset	7 years
Patents and developed technology | Minimum
Acquired Finite-Lived Intangible Assets [Line Items]
Estimated useful life of definite-lived intangible asset	2 years
Patents and developed technology | Maximum
Acquired Finite-Lived Intangible Assets [Line Items]
Estimated useful life of definite-lived intangible asset	20 years
Media content
Acquired Finite-Lived Intangible Assets [Line Items]
Estimated useful life of definite-lived intangible asset	5 years
Trademarks and trade names | Minimum
Acquired Finite-Lived Intangible Assets [Line Items]
Estimated useful life of definite-lived intangible asset	5 years
Trademarks and trade names | Maximum
Acquired Finite-Lived Intangible Assets [Line Items]
Estimated useful life of definite-lived intangible asset	13 years
Service contracts and other
Acquired Finite-Lived Intangible Assets [Line Items]
Estimated useful life of definite-lived intangible asset	10 years