Basis of Presentation and Nature of Operations - Additional Information (Details) $ in Millions	3 Months Ended	9 Months Ended
	Mar. 31, 2018 USD ($)	Sep. 30, 2018 USD ($) 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		$ 1,500.0	$ 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