Significant Accounting Policies - Revenue Recognition (Details)	12 Months Ended
	Dec. 31, 2017
Deferred Revenue Arrangement [Line Items]
Percentage of refund of tuition before the conclusion of the first seven days of a course	100.00%
Minimum
Deferred Revenue Arrangement [Line Items]
Period of revenue recognition	56 days
Maximum
Deferred Revenue Arrangement [Line Items]
Period of revenue recognition	112 days