Indebtedness - Schedule of Outstanding Indebtedness (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Outstanding indebtedness	$ 276,000	$ 856,940
Unamortized net premiums, discounts and deferred financing fees	(1,045)	(8,362)
Total	$ 274,955	848,578
Unsecured revolving credit facility, at LIBOR plus a premium
Debt Instrument [Line Items]
Interest rate stated percentage	0.00%
Outstanding indebtedness	$ 0	0
5-year unsecured term loan, at LIBOR plus a premium
Debt Instrument [Line Items]
Interest rate stated percentage	0.00%
Outstanding indebtedness	$ 0	200,000
7-year unsecured term loan, at LIBOR plus a premium
Debt Instrument [Line Items]
Interest rate stated percentage	0.00%
Outstanding indebtedness	$ 0	200,000
Unsecured floating rate debt
Debt Instrument [Line Items]
Interest rate stated percentage	0.00%
Outstanding indebtedness	$ 0	400,000
5.875% Senior Unsecured Notes due 2020
Debt Instrument [Line Items]
Interest rate stated percentage	5.875%
Outstanding indebtedness	$ 250,000	250,000
5.75% Senior Unsecured Notes due 2042
Debt Instrument [Line Items]
Interest rate stated percentage	0.00%
Outstanding indebtedness	$ 0	175,000
Unsecured fixed rate debt
Debt Instrument [Line Items]
Interest rate stated percentage	5.875%
Outstanding indebtedness	$ 250,000	425,000
Secured fixed rate debt
Debt Instrument [Line Items]
Interest rate stated percentage	5.69%
Outstanding indebtedness	$ 26,000	31,940
Secured fixed rate debt \| 206 East 9th Street
Debt Instrument [Line Items]
Interest rate stated percentage	5.69%
Outstanding indebtedness	$ 26,000	26,536
Secured fixed rate debt \| 97 Newberry Road
Debt Instrument [Line Items]
Interest rate stated percentage	0.00%
Outstanding indebtedness	$ 0	$ 5,404

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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