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Business Combinations - Narrative (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Apr. 05, 2017
Sep. 30, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Mar. 31, 2018
Business Acquisition [Line Items]            
Acquisition of CEC, net of cash acquired     $ 0 $ 353,556 $ 0  
Goodwill     $ 776,359 778,951 $ 615,433  
Community Education Centers            
Business Acquisition [Line Items]            
Percentage of voting interests acquired 100.00%          
Acquisition of CEC, net of cash acquired $ 353,600          
Operating expenses   $ 2,300        
Goodwill       $ 165,656   $ 163,107