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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Contractual interest rate for funds borrowed, under the debt agreement.

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Percentage of principal amount of debt redeemed.

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Amount of unamortized debt discount (premium) and debt issuance costs.

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Gross amount of debt extinguished.

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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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Indebtedness - Schedule of Outstanding Indebtedness (Details) - USD ($)	Jan. 30, 2018	Jul. 15, 2017	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Outstanding indebtedness			$ 856,940,000	$ 1,151,634,000
Unamortized net premiums, discounts and deferred financing fees			(8,362,000)	(9,967,000)
Total			$ 848,578,000	1,141,667,000
Unsecured revolving credit facility, at LIBOR plus a premium
Debt Instrument [Line Items]
Interest rate stated percentage			2.61%
Outstanding indebtedness			$ 0	0
5-year unsecured term loan, at LIBOR plus a premium
Debt Instrument [Line Items]
Interest rate stated percentage			2.71%
Outstanding indebtedness			$ 200,000,000	200,000,000
Total			$ 200,000,000
7-year unsecured term loan, at LIBOR plus a premium
Debt Instrument [Line Items]
Interest rate stated percentage			3.11%
Outstanding indebtedness			$ 200,000,000	200,000,000
Unsecured floating rate debt
Debt Instrument [Line Items]
Interest rate stated percentage			2.91%
Outstanding indebtedness			$ 400,000,000	400,000,000
6.65% Senior Unsecured Notes due 2018
Debt Instrument [Line Items]
Interest rate stated percentage		6.65%	0.00%
Outstanding indebtedness			$ 0	250,000,000
Amount of debt intended to be redeemed		$ 250,000,000
5.875% Senior Unsecured Notes due 2020
Debt Instrument [Line Items]
Interest rate stated percentage			5.875%
Outstanding indebtedness			$ 250,000,000	250,000,000
5.75% Senior Unsecured Notes due 2042
Debt Instrument [Line Items]
Interest rate stated percentage			5.75%
Outstanding indebtedness			$ 175,000,000	175,000,000
5.75% Senior Unsecured Notes due 2042 \| Subsequent event
Debt Instrument [Line Items]
Interest rate stated percentage	5.75%
Amount of debt intended to be redeemed	$ 175,000,000
Percentage of principal amount intended to be redeemed	100.00%
Unsecured fixed rate debt
Debt Instrument [Line Items]
Interest rate stated percentage			5.82%
Outstanding indebtedness			$ 425,000,000	675,000,000
Secured Fixed Rate Debt
Debt Instrument [Line Items]
Interest rate stated percentage			5.69%
Outstanding indebtedness			$ 31,940,000	76,634,000
Secured Fixed Rate Debt \| Parkshore Plaza
Debt Instrument [Line Items]
Interest rate stated percentage			0.00%
Outstanding indebtedness			$ 0	41,275,000
Secured Fixed Rate Debt \| 206 East 9th Street
Debt Instrument [Line Items]
Interest rate stated percentage			5.69%
Outstanding indebtedness			$ 26,536,000	27,041,000
Secured Fixed Rate Debt \| 33 Stiles Lane
Debt Instrument [Line Items]
Interest rate stated percentage			0.00%
Outstanding indebtedness			$ 0	2,415,000
Secured Fixed Rate Debt \| 97 Newberry Road
Debt Instrument [Line Items]
Interest rate stated percentage			5.71%
Outstanding indebtedness			$ 5,404,000	$ 5,903,000