Legal and Regulatory Matters - Brazilian Regulation (Details) $ in Thousands	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 ($)	Dec. 31, 2017 USD ($) educational_institution	Dec. 31, 2016 USD ($)
Loss Contingencies [Line Items]
Revenues	$ 913,710	$ 787,102	$ 1,017,196	$ 632,216	$ 951,189	$ 818,601	$ 1,017,108	$ 598,978	$ 3,350,224	$ 3,385,876	$ 3,301,864
Brazil
Loss Contingencies [Line Items]
Number of postsecondary educational institutions (educational institution) | educational_institution										13
Percentage of students enrolled in government programs									11.00%
Percentage of funds received									20.00%
Revenues									$ 654,070	$ 765,358	690,377
Grant | Brazil
Loss Contingencies [Line Items]
Revenues									$ 112,500	$ 115,200	$ 83,900