Significant Accounting Policies - Components of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Gross Profit [Abstract]
Gross revenue									$ 5,307,958	$ 5,034,199	$ 4,993,769
Less: Discounts / waivers / scholarships									(929,969)	(790,007)	(702,110)
Total	$ 1,261,223	$ 983,394	$ 1,277,439	$ 855,933	$ 1,175,893	$ 929,855	$ 1,231,910	$ 906,534	4,377,989	4,244,192	4,291,659
Tuition and educational services
Gross Profit [Abstract]
Gross revenue									4,915,415	4,640,159	4,562,704
Student fees
Gross Profit [Abstract]
Gross revenue									137,525	126,510	129,521
Dormitory / residency
Gross Profit [Abstract]
Gross revenue									29,481	65,644	75,759
Other
Gross Profit [Abstract]
Gross revenue									$ 225,537	$ 201,886	$ 225,785
Product Concentration Risk | Gross revenue
Revenue, Percentage Of Total Revenue [Abstract]
Concentration risk									121.00%	119.00%	116.00%
Product Concentration Risk | Gross revenue | Tuition and educational services
Revenue, Percentage Of Total Revenue [Abstract]
Concentration risk									112.00%	109.00%	106.00%
Product Concentration Risk | Gross revenue | Student fees
Revenue, Percentage Of Total Revenue [Abstract]
Concentration risk									3.00%	3.00%	3.00%
Product Concentration Risk | Gross revenue | Dormitory / residency
Revenue, Percentage Of Total Revenue [Abstract]
Concentration risk									1.00%	2.00%	2.00%
Product Concentration Risk | Gross revenue | Other
Revenue, Percentage Of Total Revenue [Abstract]
Concentration risk									5.00%	5.00%	5.00%
Product Concentration Risk | Discounts / waivers / scholarships
Revenue, Percentage Of Total Revenue [Abstract]
Concentration risk									21.00%	19.00%	16.00%
Product Concentration Risk | Net revenue
Revenue, Percentage Of Total Revenue [Abstract]
Concentration risk									100.00%	100.00%	100.00%