Basis of Presentation and Nature of Operations - Additional Information (Details) $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($) Office Segment	Jan. 01, 2018 USD ($)
Basis of Presentation and Nature of Operations [Line Items]
Number of operating segments | Segment	1
Number of reportable segments | Segment	1
Contract payment term description	Contracts typically contain billing terms that provide for invoicing once a month and payment on a net 30-day basis.
Contracts receivable, term	1 month
Contracts payment, term	30 days
Unfulfilled performance obligation	$ 1,200.0	$ 1,100.0
New Accounting Pronouncement Early Adoption Effect | ASU 2018-02
Basis of Presentation and Nature of Operations [Line Items]
Reclassification of tax effects from accumulated other comprehensive loss to retained earnings	0.8
Minimum | ASU 2016-02
Basis of Presentation and Nature of Operations [Line Items]
Right-of-use assets and lease liabilities	100.0
Maximum | ASU 2016-02
Basis of Presentation and Nature of Operations [Line Items]
Right-of-use assets and lease liabilities	$ 200.0
Domestic | Minimum
Basis of Presentation and Nature of Operations [Line Items]
Number of offices | Office	55
International | Minimum
Basis of Presentation and Nature of Operations [Line Items]
Number of offices | Office	15