Significant Accounting Policies - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 14, 2020	Aug. 10, 2020	Sep. 30, 2020	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Accounting Policies [Abstract]
Retained earnings			$ (188,392)	$ (188,392)		$ (152,819)
Accounting standards update						us-gaap:AccountingStandardsUpdate201613Member
Unpaid obligations record as restricted assets			347	347	$ 2	$ 300
Minimum protective endowment held in an interest-bearing account			$ 500	$ 500	500
Common stock, shares authorized (in shares)			32,000,000	32,000,000		32,000,000
Common stock, par value (in dollars per share)			$ 0.01	$ 0.01		$ 0.01
Common stock, shares issued (in shares)			24,403,364	24,403,364		21,964,809
Common stock, shares outstanding (in shares)			24,403,364	24,403,364		21,964,809
Net proceeds from issuance of common stock		$ 220,200		$ 220,248	$ 0
Underwriting discounts and offering costs		$ 9,200
Preferred stock, shares authorized (in shares)			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 dollars per share)	$ 0.60
Accumulated other comprehensive income net of tax			$ 760	$ 760		$ 233
Tax from unrealized gains and losses on marketable securities				287		90
Reclassifications from AOCI				25
Reclassifications from AOCI, tax				$ 10
Common Stock
Accounts, Notes, Loans and Financing Receivable [Line Items]
Issuance of common stock (in shares)		2,185,000	2,185,000	2,185,000
Other Assets
Accounts, Notes, Loans and Financing Receivable [Line Items]
Tuition receivable, noncurrent			$ 3,200	$ 3,200		1,000
Cumulative Effect Period of Adoption Adjustment
Accounting Policies [Abstract]
Retained earnings						$ 3,310