Debt and Financial Instruments - Additional Information (Detail)			1 Months Ended		3 Months Ended		9 Months Ended
	Jul. 02, 2021 USD ($)	Mar. 27, 2018 USD ($) Option	Mar. 31, 2020 USD ($) Derivative	Jan. 31, 2020 USD ($) Derivative	Sep. 30, 2021 USD ($)	Sep. 30, 2019 USD ($) Derivative	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Debt Instrument [Line Items]
Net borrowings on line of credit							$ 40,600,000	$ 14,150,000
Proceeds from lines of credit							40,600,000	$ 14,150,000
Outstanding borrowings under revolving credit agreement					$ 276,800,000		276,800,000		$ 236,200,000
Letters of credit outstanding					3,600,000		3,600,000
Available borrowing capacity					94,600,000		94,600,000		108,200,000
Compensating balance					0		0
Interest Rate Swap
Debt Instrument [Line Items]
Derivative interest rate cap, net payment received or accrued from counterparties							945,000
Interest Rate Swap | Cash Flow Hedge
Debt Instrument [Line Items]
Number of interest rate cap agreements | Derivative			1	1		1
Derivative instruments, fixed rate			0.565%	1.4975%		1.144%
Notional amount			$ 55,000,000	$ 35,000,000		$ 50,000,000
Expiration date of interest rate			Mar. 25, 2027	Sep. 16, 2024		Sep. 16, 2024
Level 2
Debt Instrument [Line Items]
Mortgage loan fair value					60,300,000		60,300,000		62,000,000.0
Mortgage debt									$ 59,200,000
Derivative interest rate cap, net payment received or accrued from counterparties							1,600,000
Derivative interest rate cap, payment received or accrued from counterparties							199,000
Derivative interest rate cap, offset due to counterparties							1,800,000
Level 2 | Interest Rate Swap
Debt Instrument [Line Items]
Liability derivatives, fair value					556,000		$ 556,000
Derivative interest rate cap, net payment received or accrued from counterparties					$ 324,000
London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Margin points added to the reference rate		1.25%
Base Rate
Debt Instrument [Line Items]
Margin points added to the reference rate		0.25%
Credit Agreement
Debt Instrument [Line Items]
Outstanding borrowing	$ 350,000,000	$ 375,000,000
Net borrowings on line of credit	350,000,000	$ 50,000,000
Unsecured revolving amended credit agreement terminated date		Jul. 02, 2025
Proceeds from lines of credit	350,000,000	$ 50,000,000
Increase in borrowing capacity	$ 375,000,000
Number of additional six month extension options | Option		2
Credit Agreement | Swingline/Short-Term Loans
Debt Instrument [Line Items]
Outstanding borrowing		$ 30,000,000
Credit Agreement | Letters of Credit
Debt Instrument [Line Items]
Outstanding borrowing		$ 40,000,000
Revolving A Facility
Debt Instrument [Line Items]
Credit facility, Interest Rate Terms							Borrowings under the Credit Agreement will bear interest annually at a rate equal to, at our option, at either LIBOR (for one, three, or six months) or the Base Rate, plus in either case, a specified margin depending on our ratio of debt to total capital, as determined by the formula set forth in the Credit Agreement. The applicable margin ranges from 1.10% to 1.35% for LIBOR loans and 0.10% to 0.35% for Base Rate loans.
Base rate description							the greatest of (a) the Administrative Agent’s prime rate, (b) the federal funds effective rate plus 1/2 of 1% and (c) one month LIBOR plus 1%.
Revolving A Facility | Minimum
Debt Instrument [Line Items]
Facility fee payable on commitment		0.15%
Revolving A Facility | Minimum | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Margin points added to the reference rate		1.10%
Margin points added to the base rate		1.00%
Revolving A Facility | Minimum | Base Rate
Debt Instrument [Line Items]
Margin points added to the reference rate		0.10%
Revolving A Facility | Minimum | Federal Funds Effective Rate
Debt Instrument [Line Items]
Margin points added to the base rate		0.50%
Revolving A Facility | Maximum
Debt Instrument [Line Items]
Facility fee payable on commitment		0.35%
Revolving A Facility | Maximum | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Margin points added to the reference rate		1.35%
Revolving A Facility | Maximum | Base Rate
Debt Instrument [Line Items]
Margin points added to the reference rate		0.35%
Revolving B Facility
Debt Instrument [Line Items]
Facility fee payable on commitment							0.25%
Revolving B Facility | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Margin points added to the reference rate							1.25%
Revolving B Facility | Base Rate
Debt Instrument [Line Items]
Margin points added to the reference rate							0.25%