Debt - Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Long-term debt, gross	$ 1,602,600	$ 1,651,085
Unamortized fair market value premium	31	39
Unamortized debt issuance costs	(6,389)	(6,111)
Principal outstanding	1,596,242	1,645,013
Long-term debt, fair value	1,649,260	1,691,514
Unsecured Credit Facility
Debt Instrument [Line Items]
Principal outstanding	0	146,000
Long-term debt, fair value	0	146,000
Unsecured Term Loans
Debt Instrument [Line Items]
Long-term debt, gross	975,000	875,000
Unamortized debt issuance costs	(4,304)	(3,625)
Principal outstanding	970,696	871,375
Long-term debt, fair value	965,296	875,000
Unsecured Notes
Debt Instrument [Line Items]
Long-term debt, gross	575,000	575,000
Unamortized debt issuance costs	(1,819)	(2,117)
Principal outstanding	573,181	572,883
Long-term debt, fair value	628,955	614,493
Mortgage Loans Payable
Debt Instrument [Line Items]
Long-term debt, gross	52,600	55,085
Unamortized fair market value premium	31	39
Unamortized debt issuance costs	(266)	(369)
Principal outstanding	52,365	54,755
Long-term debt, fair value	$ 55,009	$ 56,021

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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount, after accumulated amortization, of debt premium.

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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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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

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