Description of Business and Significant Accounting Policies - Concentrations of Credit Risk and Significant Clients (Details) $ in Thousands	3 Months Ended								12 Months Ended
	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%