Business Combinations, Goodwill and Intangible Assets - Weighted Average Life of Intangible Assets (Details)	12 Months Ended
	Dec. 31, 2017
Customer relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Weighted average life of intangible assets	5 years 4 months 10 days
Leasehold benefits
Acquired Finite-Lived Intangible Assets [Line Items]
Weighted average life of intangible assets	1 year 4 months 28 days
Developed technology
Acquired Finite-Lived Intangible Assets [Line Items]
Weighted average life of intangible assets	3 years 24 days
Non-compete agreements
Acquired Finite-Lived Intangible Assets [Line Items]
Weighted average life of intangible assets	1 year 8 months 9 days
Trade names and trademarks
Acquired Finite-Lived Intangible Assets [Line Items]
Weighted average life of intangible assets	4 years 3 months 15 days