Significant Accounting Policies - Narrative (Details)		1 Months Ended	3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 13, 2021 $ / shares	Jul. 31, 2021 $ / shares	Sep. 30, 2021 USD ($) uSHigherEducationCampus $ / shares shares	Sep. 30, 2020 $ / shares	Jun. 30, 2021 USD ($) uSHigherEducationCampus	Sep. 30, 2021 USD ($) $ / shares shares	Sep. 30, 2020 USD ($) $ / shares	Dec. 31, 2020 USD ($) $ / shares shares
Long Lived Assets Held-for-sale [Line Items]
Number of long-lived assets marketed for sale | uSHigherEducationCampus					2
Impairment of fixed asset impairment charges			$ 600,000			$ 2,600,000
Number of long-lived assets sold | uSHigherEducationCampus			1
Gain on sale of asset			$ 700,000
Common stock, shares authorized (in shares) | shares			32,000,000			32,000,000		32,000,000
Common stock, par value (in dollars per share) | $ / shares			$ 0.01			$ 0.01		$ 0.01
Common stock, shares issued (in shares) | shares			24,600,479			24,600,479		24,418,939
Common stock, shares outstanding (in shares) | shares			24,600,479			24,600,479		24,418,939
Preferred stock, shares authorized (in shares) | shares			8,000,000			8,000,000		8,000,000
Issued shares of preferred stock (in shares) | shares			0			0		0
Outstanding shares of preferred stock (in shares) | shares			0			0		0
Common stock dividends declared (in dollars per share) | $ / shares		$ 0.60	$ 0.60	$ 0.60		$ 1.80	$ 1.80
Common stock dividends paid (in dollars per share) | $ / shares	$ 0.60
Accumulated other comprehensive income net of tax			$ 1,956,000			$ 1,956,000		$ 48,880,000
Tax from unrealized gains and losses on marketable securities						300,000		$ 300,000
Reclassifications from AOCI			$ 0			$ 0	$ 25,000
Reclassifications from AOCI, tax							$ 10,000
Two U.S. Higher Education Campus Locations
Long Lived Assets Held-for-sale [Line Items]
Impairment of fixed asset impairment charges					$ 0