Revenue - Narrative (Details) - USD ($)	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Accounts and notes receivable	$ 571,240,000	$ 474,456,000
Deferred revenue and student deposits, current	465,290,000	184,116,000
Revenue recognized	166,000,000
Capitalized contract cost	8,300,000	0
Capitalized contract cost, accumulated amortization	3,400,000	$ 0
Amortization of capitalized costs	$ 3,400,000
Minimum
Disaggregation of Revenue [Line Items]
Contract cost, amortization period	2 years
Maximum
Disaggregation of Revenue [Line Items]
Contract cost, amortization period	4 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction	12 months
Commission And Bonuses
Disaggregation of Revenue [Line Items]
Remaining performance obligations recognition period	one year or less
Incremental Costs Of Obtaining A Contract With Customer
Disaggregation of Revenue [Line Items]
Remaining performance obligations recognition period	one year or less
Incremental Cost Of Obtaining A Contract With Students
Disaggregation of Revenue [Line Items]
Remaining performance obligations recognition period	one year or less