Debt - Schedule of Maturity Amounts Outstanding Under Credit Agreement and Senior Unsecured Notes (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Line of Credit Facility [Line Items]
Borrowings under credit agreement, outstanding amount		$ 120,000
Borrowings under note purchase and guarantee agreement, outstanding amount	$ 449,024	324,409
Total	450,000	445,000
Unamortized debt issuance costs, net	(3,210)	(3,364)
Total debt, net	$ 446,790	$ 441,636
Revolving Facility [Member]
Line of Credit Facility [Line Items]
Maturity Date	Mar. 23, 2022	Mar. 23, 2022
Borrowings under credit agreement, outstanding amount		$ 70,000
Total	$ 0
Term Loan [Member]
Line of Credit Facility [Line Items]
Maturity Date	Mar. 23, 2023	Mar. 23, 2023
Borrowings under credit agreement, outstanding amount		$ 50,000
Series A Notes Maturing in February 2021 [Member]
Line of Credit Facility [Line Items]
Maturity Date	Feb. 25, 2021	Feb. 28, 2021
Interest Rate	6.00%	6.00%
Borrowings under note purchase and guarantee agreement, outstanding amount	$ 100,000	$ 100,000
Series B Notes Maturing in June 2023 [Member]
Line of Credit Facility [Line Items]
Maturity Date	Jun. 02, 2023	Jun. 30, 2023
Interest Rate	5.35%	5.35%
Borrowings under note purchase and guarantee agreement, outstanding amount	$ 75,000	$ 75,000
Series C Notes Maturing in February 2025 [Member]
Line of Credit Facility [Line Items]
Maturity Date	Feb. 25, 2025	Feb. 28, 2025
Interest Rate	4.75%	4.75%
Borrowings under note purchase and guarantee agreement, outstanding amount	$ 50,000	$ 50,000
Series D Notes Maturing in June 2028 [Member]
Line of Credit Facility [Line Items]
Maturity Date	Jun. 21, 2028	Jun. 21, 2028
Interest Rate	5.47%	5.47%
Borrowings under note purchase and guarantee agreement, outstanding amount	$ 50,000	$ 50,000
Series E Notes Maturing in June 2028 [Member]
Line of Credit Facility [Line Items]
Maturity Date	Jun. 21, 2028	Jun. 21, 2028
Interest Rate	5.47%	5.47%
Borrowings under note purchase and guarantee agreement, outstanding amount	$ 50,000	$ 50,000
Series F Notes Maturing in September 2029 [Member]
Line of Credit Facility [Line Items]
Maturity Date	Sep. 12, 2029	Sep. 30, 2029
Interest Rate	3.52%	3.52%
Borrowings under note purchase and guarantee agreement, outstanding amount	$ 50,000
Series G Notes Maturing in September 2029 [Member]
Line of Credit Facility [Line Items]
Maturity Date	Sep. 12, 2029	Sep. 30, 2029
Interest Rate	3.52%	3.52%
Borrowings under note purchase and guarantee agreement, outstanding amount	$ 50,000
Series H Note Maturing In September Two Thousand Twenty Nine
Line of Credit Facility [Line Items]
Maturity Date	Sep. 12, 2029	Sep. 30, 2029
Interest Rate	3.52%	3.52%
Borrowings under note purchase and guarantee agreement, outstanding amount	$ 25,000