Summary of Significant Accounting Policies - Additional Information (Details)	3 Months Ended									12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	[1]	Dec. 31, 2017 USD ($) Segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Summary Of Significant Accounting Policies [Line Items]
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	321,174,000	$ 305,301,000	$ 306,392,000	$ 296,295,000	$ 289,559,000	$ 306,520,000	$ 305,419,000	$ 283,599,000		$ 1,229,162,000	$ 1,185,097,000	$ 1,132,232,000
Foreign financial institutions, actual deposits	$ 10,400,000				$ 5,400,000					10,400,000	5,400,000
Federal Government Agencies And Departments
Summary Of Significant Accounting Policies [Line Items]
Revenue										$ 550,300,000	$ 563,000,000	$ 540,000,000
Federal Government Agencies And Departments | Customer Concentration Risk | Sales Revenue, Net
Summary Of Significant Accounting Policies [Line Items]
Concentration risk, percentage										45.00%	48.00%	48.00%
International Clients | Geographic Concentration Risk | Sales Revenue, Net
Summary Of Significant Accounting Policies [Line Items]
Concentration risk, percentage										9.00%	10.00%	11.00%
Minimum
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life										2 years
Finite-lived intangible asset, useful life										4 years
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 | Other Assets
Summary Of Significant Accounting Policies [Line Items]
Finite-lived intangible asset, useful life										5 years

[1] Includes adjustments for the adoption ASU 2016-09 in the second quarter of 2016.