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Business Combinations - Narrative (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Apr. 05, 2017
Jun. 30, 2018
Mar. 31, 2018
Jun. 30, 2018
Jun. 30, 2017
Business Acquisition [Line Items]          
Acquisition of CEC, net of cash acquired       $ 0 $ 354,466
Community Education Centers          
Business Acquisition [Line Items]          
Percentage of voting interests acquired 100.00%        
Acquisition of CEC, net of cash acquired $ 353,600        
Measurement period adjustment     $ 1,000    
Measurement period adjustment, valuation of an unfavorable contract liability     $ 1,400    
Operating expenses   $ 2,300