Significant Accounting Policies - Additional Information (Details) - USD ($)		3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 16, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Jan. 01, 2019
Accounting Policies [Abstract]
Unpaid obligations record as restricted assets		$ 2,000	$ 15,000	$ 2,000	$ 15,000	$ 5,000
Minimum protective endowment held in an interest-bearing account		$ 500,000	$ 500,000	$ 500,000	$ 500,000
Authorized shares of common stock		32,000,000		32,000,000		32,000,000
Common stock, par value (in dollars per share)		$ 0.01		$ 0.01		$ 0.01
Issued shares of common stock		21,964,505		21,964,505		21,743,498
Outstanding shares of common stock		21,964,505		21,964,505		21,743,498
Authorized shares of preferred stock		8,000,000		8,000,000		8,000,000
Issued shares of preferred stock		0		0		0
Outstanding shares of preferred stock		0		0		0
Common stock dividends (in dollar per share)	$ 0.50
Amount of antidilutive securities excluded from computation of earnings per share (in shares)		3,000	681,000	21,000	583,000
Accumulated other comprehensive income net of tax		$ 447,000		$ 447,000		$ 32,000
Tax from unrealized gains and losses on marketable securities				157,000		10,000
Reclassifications from AOCI		0		0
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Lease liability		109,234,000		109,234,000
Right-of-use asset		93,208,000		93,208,000
Other Assets
Accounts, Notes, Loans and Financing Receivable [Line Items]
Tuition receivable, noncurrent		$ 1,100,000		$ 1,100,000		$ 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