Significant Accounting Policies - Additional Information (Details) - USD ($)		3 Months Ended		12 Months Ended
	Mar. 18, 2019	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Jan. 01, 2019
Accounting Policies [Abstract]
Unpaid obligations record as restricted assets		$ 5,000	$ 8,000	$ 5,000
Minimum protective endowment held in an interest-bearing account		$ 500,000	$ 500,000	$ 500,000
Authorized shares of common stock		32,000,000		32,000,000
Par value of common stock (in dollars per share)		$ 0.01		$ 0.01
Issued shares of common stock		21,923,800		21,743,498
Outstanding shares of common stock		21,923,800		21,743,498
Authorized shares of preferred stock		8,000,000		8,000,000
Issued shares of preferred stock		0		0
Outstanding shares of preferred stock		0		0
Dividends paid per share (in dollars per share)	$ 0.50
Amount of antidilutive securities excluded from computation of earnings per share (in shares)		59,000	0
Accumulated other comprehensive income net of tax		$ 266,000		$ 32,000
Tax from unrealized gains and losses on marketable securities		116,000		10,000
Reclassifications from AOCI		0
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Lease liability		116,963,000
Right-of-use asset		101,533,000
Other Assets
Accounts, Notes, Loans and Financing Receivable [Line Items]
Tuition receivable, noncurrent		$ 800,000		$ 1,100,000
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Lease liability					$ 123,000,000
Right-of-use asset					$ 107,000,000