Unsecured Line of Credit and Term Notes - Borrowings Outstanding on Unsecured Line of Credit and Term Notes (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Revolving line of credit borrowings	$ 25,000	$ 91,000
Total term note principal balance outstanding	1,875,000	1,625,000
Less: unamortized debt issuance costs	(11,611)	(9,778)
Less: unamortized senior term note discount	(5,765)	(4,402)
Term notes payable	1,857,624	1,610,820
Term Note Due June 4, 2020 [Member]
Debt Instrument [Line Items]
Total term note principal balance outstanding		100,000
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

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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