Significant Accounting Policies - Reclassification of Operation Expenses (Details) - USD ($) $ in Thousands	3 Months Ended					9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Error Corrections and Prior Period Adjustments Restatement [Line Items]
I&SC	$ 132,527	$ 130,712	$ 103,079	$ 69,299	$ 68,452	$ 397,281	$ 240,830
G&A	$ 72,303	68,541	61,976	32,176	31,342	$ 204,816	$ 125,494
Instruction and educational support | Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
I&SC		118,320	93,290	64,690	63,776
G&A		0	0	0	0
Admissions advisory | Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
I&SC		12,392	9,789	4,609	4,676
G&A		0	0	0	0
Marketing | Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
I&SC		0	0	0	0
G&A		49,577	46,165	21,113	20,124
General and administration | Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
I&SC		0	0	0	0
G&A		$ 18,964	$ 15,811	$ 11,063	$ 11,218