Business Combinations, Goodwill and Intangible Assets - Weighted Average Life of Intangible Assets (Details)	6 Months Ended
	Jun. 30, 2018
Customer relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Weighted average life of intangible assets	5 years 1 month 17 days
Leasehold benefits
Acquired Finite-Lived Intangible Assets [Line Items]
Weighted average life of intangible assets	11 months 1 day
Developed technology
Acquired Finite-Lived Intangible Assets [Line Items]
Weighted average life of intangible assets	3 years
Non-compete agreements
Acquired Finite-Lived Intangible Assets [Line Items]
Weighted average life of intangible assets	1 year 2 months 19 days
Trade names and trademarks (Finite lived)
Acquired Finite-Lived Intangible Assets [Line Items]
Weighted average life of intangible assets	4 years 2 months 12 days