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Description of Business and Significant Accounting Policies - Concentrations of Credit Risk and Significant Clients (Details) $ in Thousands | 3 Months Ended | 12 Months Ended
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Dec. 31, 2018 USD ($) financial_institution | Sep. 30, 2018 USD ($) | Jun. 30, 2018 USD ($) | Mar. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) financial_institution | Sep. 30, 2017 USD ($) | Jun. 30, 2017 USD ($) | Mar. 31, 2017 USD ($) | Dec. 31, 2018 USD ($) financial_institution | Dec. 31, 2017 USD ($) financial_institution | Dec. 31, 2016 USD ($)
Accounting Policies [Abstract] |  |  |  |  |  |  |  |  |  |  |
Number of financial institutions holding cash and cash equivalents and marketable securities | financial_institution | 1 |  |  |  | 1 |  |  |  | 1 | 1 |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue | $ 496,408 | $ 506,871 | $ 503,732 | $ 501,810 | $ 499,410 | $ 491,355 | $ 470,876 | $ 404,490 | $ 2,008,821 | $ 1,866,131 | $ 1,562,662
Customer Concentration Risk | Revenue from Contract with Customer | Genesis |  |  |  |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  | $ 386,700 | $ 327,500 |
Concentration risk, percentage |  |  |  |  |  |  |  |  | 19.30% | 17.50% |

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