Significant Accounting Policies - Goodwill and Intangible Assets (Details)	12 Months Ended
	Dec. 31, 2017
Client relationships
Goodwill and intangible assets [Line items]
Useful life	13 years
Other intangible assets
Goodwill and intangible assets [Line items]
Useful life	8 years 8 months 12 days
Minimum | Client relationships
Goodwill and intangible assets [Line items]
Useful life	3 years
Minimum | Other intangible assets
Goodwill and intangible assets [Line items]
Useful life	3 years
Maximum | Client relationships
Goodwill and intangible assets [Line items]
Useful life	19 years
Maximum | Other intangible assets
Goodwill and intangible assets [Line items]
Useful life	19 years