Significant Accounting Policies - Recent Accounting Pronouncements (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment of retained earnings		$ (108,569)	$ (87,448)
Income tax expense		11,493	14,940	$ 20,072
Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Graduation fee revenue		1,100
Revenue, performance obligation not satisfied		400
Adjustment of retained earnings		400
Accounting Standards Update 2015-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred tax asset, current			5,100
Deferred tax liabilities, noncurrent			$ 13,900
Accounting Standards Update 2016-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Income tax expense		$ 500
Accounting Standards Update 2016-09 | Minimum | Scenario, Forecast
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Income tax expense	$ 200
Accounting Standards Update 2016-09 | Maximum | Scenario, Forecast
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Income tax expense	$ 400