Unsecured Line of Credit and Term Notes - Borrowings Outstanding on Unsecured Line of Credit and Term Notes (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Revolving line of credit borrowings		$ 65,000
Total term note principal balance outstanding	$ 2,275,000	1,875,000
Less: unamortized debt issuance costs	(13,277)	(11,146)
Less: unamortized senior term note discount	(6,940)	(5,583)
Term notes payable	2,254,783	1,858,271
Term Note Due August 5, 2021 [Member]
Debt Instrument [Line Items]
Total term note principal balance outstanding	100,000	100,000
Term Note Due April 8, 2024 [Member]
Debt Instrument [Line Items]
Total term note principal balance outstanding	175,000	175,000
Senior Term Note Due July 1, 2026 [Member]
Debt Instrument [Line Items]
Total term note principal balance outstanding	600,000	600,000
Senior Term Note Due December 15, 2027 [Member]
Debt Instrument [Line Items]
Total term note principal balance outstanding	450,000	450,000
Term Note Due July 21, 2028 [Member]
Debt Instrument [Line Items]
Total term note principal balance outstanding	200,000	200,000
Senior Term Note Due June 15, 2029 [Member]
Debt Instrument [Line Items]
Total term note principal balance outstanding	350,000	$ 350,000
Senior Term Note Due October 15, 2030 [Member]
Debt Instrument [Line Items]
Total term note principal balance outstanding	$ 400,000

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Amount, after accumulated amortization, of debt discount.

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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