Summary of Significant Accounting Policies - Additional Information (Details)	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) Segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 01, 2018 USD ($)
Summary Of Significant Accounting Policies [Line Items]
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
Number of reportable segments | Segment									1
Goodwill, impaired, accumulated impairment loss	$ 0								$ 0
Goodwill, impairment loss									$ 0
Performance based share awards, performance period									3 years
Number of operating segments | Segment									1
Revenue	377,910,000	$ 332,968,000	$ 324,315,000	$ 302,780,000	$ 321,174,000	$ 305,301,000	$ 306,392,000	$ 296,295,000	$ 1,337,973,000	$ 1,229,162,000	$ 1,185,097,000
Foreign financial institutions, actual deposits	11,900,000				$ 10,400,000				11,900,000	10,400,000
Unfulfilled performance obligation												$ 1,100,000,000
New Accounting Pronouncement Early Adoption Effect | ASU 2018-02
Summary Of Significant Accounting Policies [Line Items]
Reclassification of tax effects from accumulated other comprehensive loss to retained earnings				$ 800,000
Federal Government Agencies And Departments
Summary Of Significant Accounting Policies [Line Items]
Revenue									$ 543,900,000	$ 550,300,000	$ 563,000,000
Federal Government Agencies And Departments | Customer Concentration Risk | Sales Revenue, Net
Summary Of Significant Accounting Policies [Line Items]
Concentration risk, percentage									41.00%	45.00%	48.00%
International Clients | Geographic Concentration Risk | Sales Revenue, Net
Summary Of Significant Accounting Policies [Line Items]
Concentration risk, percentage									15.00%	9.00%	10.00%
International | Long-Lived Assets
Summary Of Significant Accounting Policies [Line Items]
Concentration risk, percentage									12.20%	7.90%
Minimum
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life									2 years
Finite-lived intangible asset, useful life									1 year
Minimum | ASU 2016-02
Summary Of Significant Accounting Policies [Line Items]
Right-of-use asset	140,000,000								$ 140,000,000
Lease liability	155,000,000								$ 155,000,000
Minimum | Other Assets
Summary Of Significant Accounting Policies [Line Items]
Finite-lived intangible asset, useful life									3 years
Maximum
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life									7 years
Finite-lived intangible asset, useful life									10 years
Maximum | ASU 2016-02
Summary Of Significant Accounting Policies [Line Items]
Right-of-use asset	165,000,000								$ 165,000,000
Lease liability	$ 180,000,000								$ 180,000,000
Maximum | Other Assets
Summary Of Significant Accounting Policies [Line Items]
Finite-lived intangible asset, useful life									5 years