Debt - Schedule of Maturity Amounts Outstanding Under Credit Agreement and Senior Unsecured Notes (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Dec. 14, 2020
Line of Credit Facility [Line Items]
Borrowings under credit agreement, outstanding amount		$ 25,000,000
Borrowings under note purchase and guarantee agreement, outstanding amount	$ 523,868,000	523,828,000
Total debt	525,000,000	550,000,000
Unamortized debt issuance costs, net	(2,047,000)	(2,307,000)
Total debt, net	$ 522,953,000	$ 547,693,000
Revolving Facility [Member]
Line of Credit Facility [Line Items]
Maturity Date	Mar. 31, 2022	Mar. 31, 2022
Borrowings under credit agreement, outstanding amount	$ 75,000,000	$ 25,000,000	$ 75,000,000
Series B Notes Maturing in June 2023 [Member]
Line of Credit Facility [Line Items]
Maturity Date	Jun. 30, 2023	Jun. 30, 2023
Interest Rate	5.35%	5.35%
Borrowings under note purchase and guarantee agreement, outstanding amount	$ 75,000,000	$ 75,000,000
Series C Notes Maturing in February 2025 [Member]
Line of Credit Facility [Line Items]
Maturity Date	Feb. 28, 2025	Feb. 28, 2025
Interest Rate	4.75%	4.75%
Borrowings under note purchase and guarantee agreement, outstanding amount	$ 50,000,000	$ 50,000,000
Series D Notes Maturing in June 2028 [Member]
Line of Credit Facility [Line Items]
Maturity Date	Jun. 30, 2028	Jun. 30, 2028
Interest Rate	5.47%	5.47%
Borrowings under note purchase and guarantee agreement, outstanding amount	$ 50,000,000	$ 50,000,000
Series E Notes Maturing in June 2028 [Member]
Line of Credit Facility [Line Items]
Maturity Date	Jun. 30, 2028	Jun. 30, 2028
Interest Rate	5.47%	5.47%
Borrowings under note purchase and guarantee agreement, outstanding amount	$ 50,000,000	$ 50,000,000
Series F Notes Maturing in September 2029 [Member]
Line of Credit Facility [Line Items]
Maturity Date	Sep. 30, 2029	Sep. 30, 2029
Interest Rate	3.52%	3.52%
Borrowings under note purchase and guarantee agreement, outstanding amount	$ 50,000,000	$ 50,000,000
Series G Notes Maturing in September 2029 [Member]
Line of Credit Facility [Line Items]
Maturity Date	Sep. 30, 2029	Sep. 30, 2029
Interest Rate	3.52%	3.52%
Borrowings under note purchase and guarantee agreement, outstanding amount	$ 50,000,000	$ 50,000,000
Series I Notes Maturing in November 2030 [Member]
Line of Credit Facility [Line Items]
Maturity Date	Nov. 30, 2030	Nov. 30, 2030
Interest Rate	3.43%	3.43%
Borrowings under note purchase and guarantee agreement, outstanding amount	$ 100,000,000	$ 100,000,000
Series H Notes Maturing in September 2029 [Member]
Line of Credit Facility [Line Items]
Maturity Date	Sep. 30, 2029	Sep. 30, 2029
Interest Rate	3.52%	3.52%
Borrowings under note purchase and guarantee agreement, outstanding amount	$ 25,000,000	$ 25,000,000
Series J Notes Maturing in November 2030 [Member]
Line of Credit Facility [Line Items]
Maturity Date	Nov. 30, 2030	Nov. 30, 2030
Interest Rate	3.43%	3.43%
Borrowings under note purchase and guarantee agreement, outstanding amount	$ 50,000,000	$ 50,000,000
Series K Notes Maturing in November 2030 [Member]
Line of Credit Facility [Line Items]
Maturity Date	Nov. 30, 2030	Nov. 30, 2030
Interest Rate	3.43%	3.43%
Borrowings under note purchase and guarantee agreement, outstanding amount	$ 25,000,000	$ 25,000,000