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Summary of Significant Accounting Policies - Additional Information (Details) | 3 Months Ended | 12 Months Ended
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Dec. 31, 2020 USD ($) | Sep. 30, 2020 USD ($) | Jun. 30, 2020 USD ($) | Mar. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Sep. 30, 2019 USD ($) | Jun. 30, 2019 USD ($) | Mar. 31, 2019 USD ($) | Dec. 31, 2020 USD ($) Segment | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($)
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |
Contract payment term description |  |  |  |  |  |  |  |  | contracts typically contain billing terms that provide for invoicing monthly or upon completion of milestones, and payment on a net 30-day basis. |  |
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 | 434,335,000 | $ 360,315,000 | $ 353,987,000 | $ 358,238,000 | $ 396,636,000 | $ 373,918,000 | $ 366,717,000 | $ 341,254,000 | $ 1,506,875,000 | $ 1,478,525,000 | $ 1,337,973,000
Foreign financial institutions, actual deposits | 81,500,000 |  |  |  | $ 7,100,000 |  |  |  | 81,500,000 | 7,100,000 |
Federal Government Agencies And Departments |  |  |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  | $ 667,000,000.0 | $ 561,000,000.0 | $ 546,500,000
Federal Government Agencies And Departments | Customer Concentration Risk | Sales Revenue, Net |  |  |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |
Concentration risk, percentage |  |  |  |  |  |  |  |  | 44.00% | 38.00% | 41.00%
International Clients | Geographic Concentration Risk | Sales Revenue, Net |  |  |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |
Concentration risk, percentage |  |  |  |  |  |  |  |  | 13.00% | 8.00% | 13.00%
International | Long-Lived Assets |  |  |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |
Concentration risk, percentage |  |  |  |  |  |  |  |  | 15.00% | 17.00% |
Indirect and Selling Expenses |  |  |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |
Impairment expense related to intangible assets |  |  |  |  |  |  |  |  | $ 3,100,000 | $ 1,700,000 |
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 | 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 |  |
Insured domestic bank accounts | $ 250,000 |  |  |  |  |  |  |  | $ 250,000 |  |
Maximum | Other Assets |  |  |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |
Finite-lived intangible asset, useful life |  |  |  |  |  |  |  |  | 5 years |  |

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- Definition For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 42 -URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e9054-108599 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6351-108592 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13531-108611 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 21 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13537-108611 Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592 + Details
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