Debt (Schedule of Debt) (Details)	12 Months Ended
Dec. 31, 2018 USD ($) period	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Subtotal/Weighted Average	3.76%	3.48%
Amount Outstanding	$ 1,685,472,000	$ 1,726,927,000
Secured Debt
Debt Instrument [Line Items]
Subtotal/Weighted Average	3.80%
Amount Outstanding	$ 190,351,000	191,616,000
Premium (discount) and unamortized debt issuance costs	$ 645,000	946,000
Secured Debt \| Secured 35 Million Mortgage Loan
Debt Instrument [Line Items]
Stated Rate	5.55%
Effective Rate	3.75%
Amount Outstanding	$ 29,706,000	30,670,000
Face amount of facility	$ 35,000,000	35,000,000
Secured Debt \| Fixed Rate 160 Million Loan
Debt Instrument [Line Items]
Stated Rate	3.48%
Effective Rate	3.58%
Amount Outstanding	$ 160,000,000	160,000,000
Face amount of facility	$ 160,000,000
Unsecured Debt
Debt Instrument [Line Items]
Subtotal/Weighted Average	3.75%
Amount Outstanding	$ 1,495,121,000	1,535,311,000
Premium (discount) and unamortized debt issuance costs	(9,879,000)	(7,689,000)
Derivative, notional amount	$ 450,000,000
Unsecured Debt \| $170 Million Unsecured 2015 Term Loan
Debt Instrument [Line Items]
Effective Rate	2.54%
Amount Outstanding	$ 0	170,000,000
Face amount of facility	170,000,000
Unsecured Debt \| $300 Million Unsecured 2013 Term Loan
Debt Instrument [Line Items]
Amount Outstanding	0	300,000,000
Face amount of facility	300,000,000
Unsecured Debt \| Unsecured 500 Million 2018 Line Of Credit
Debt Instrument [Line Items]
Face amount of facility	500,000,000
Unsecured Debt \| Amended and Restated $300 Million Unsecured 2011 Term Loan
Debt Instrument [Line Items]
Amount Outstanding	300,000,000	300,000,000
Face amount of facility	$ 300,000,000
Additional extension period	22 months
Unsecured Debt \| Unsecured Senior 350 Million Notes
Debt Instrument [Line Items]
Stated Rate	3.40%
Effective Rate	3.43%
Amount Outstanding	$ 350,000,000	350,000,000
Face amount of facility	$ 350,000,000
Unsecured Debt \| Unsecured Senior 400 Million Notes
Debt Instrument [Line Items]
Stated Rate	4.45%
Effective Rate	4.10%
Amount Outstanding	$ 400,000,000	400,000,000
Face amount of facility	$ 400,000,000
Unsecured Debt \| $250 Million Unsecured 2018 Term Loan
Debt Instrument [Line Items]
Effective Rate	4.12%
Amount Outstanding	$ 250,000,000	0
Face amount of facility	250,000,000
Derivative, notional amount	$ 150,000,000
Line of Credit \| Unsecured Debt \| $500 Million Unsecured 2015 Line of Credit
Debt Instrument [Line Items]
Effective Rate	3.17%
Amount Outstanding	$ 0	23,000,000
Face amount of facility	$ 500,000,000
Line of Credit \| Unsecured Debt \| Unsecured 500 Million 2018 Line Of Credit
Debt Instrument [Line Items]
Effective Rate	3.35%
Amount Outstanding	$ 205,000,000	$ 0
Additional extension period	1 year
Number of extension periods \| period	2
Maturity extension period	6 months
Interest rate swaps \| $250 Million Unsecured 2018 Term Loan
Debt Instrument [Line Items]
Derivative, notional amount	$ 150,000,000
Interest rate swaps \| Seven Year Interest Rate Swap Agreement
Debt Instrument [Line Items]
Derivative, notional amount	$ 100,000,000
Interest rate swaps \| Unsecured Debt \| $300 Million Unsecured 2013 Term Loan
Debt Instrument [Line Items]
Effective Rate	2.78%
Interest rate swaps \| Unsecured Debt \| Amended and Restated $300 Million Unsecured 2011 Term Loan
Debt Instrument [Line Items]
Effective Rate	3.20%
Through March 29, 2020 \| Unsecured Debt \| $250 Million Unsecured 2018 Term Loan
Debt Instrument [Line Items]
Derivative fixed interest rate	4.11%
From March 30, 2020 Through March 31, 2025 \| Unsecured Debt \| $250 Million Unsecured 2018 Term Loan
Debt Instrument [Line Items]
Derivative fixed interest rate	4.21%
LIBOR \| $250 Million Unsecured 2018 Term Loan
Debt Instrument [Line Items]
Basis spread on variable interest rate	1.00%
LIBOR \| Unsecured Debt \| $170 Million Unsecured 2015 Term Loan
Debt Instrument [Line Items]
Basis spread on variable interest rate	1.125%
LIBOR \| Unsecured Debt \| $300 Million Unsecured 2013 Term Loan
Debt Instrument [Line Items]
Basis spread on variable interest rate	1.20%
LIBOR \| Unsecured Debt \| Unsecured 500 Million 2018 Line Of Credit
Debt Instrument [Line Items]
Basis spread on variable interest rate	0.90%
LIBOR \| Unsecured Debt \| Amended and Restated $300 Million Unsecured 2011 Term Loan
Debt Instrument [Line Items]
Basis spread on variable interest rate	1.00%
LIBOR \| Unsecured Debt \| $250 Million Unsecured 2018 Term Loan
Debt Instrument [Line Items]
Basis spread on variable interest rate	1.60%
LIBOR \| Line of Credit \| Unsecured Debt \| $500 Million Unsecured 2015 Line of Credit
Debt Instrument [Line Items]
Basis spread on variable interest rate	1.00%
LIBOR \| Line of Credit \| Unsecured Debt \| Unsecured 500 Million 2018 Line Of Credit
Debt Instrument [Line Items]
Basis spread on variable interest rate	0.90%
LIBOR \| Line of Credit \| Unsecured Debt \| Amended and Restated $300 Million Unsecured 2011 Term Loan
Debt Instrument [Line Items]
Basis spread on variable interest rate	1.00%

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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Face (par) amount of debt instrument at time of issuance.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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Contractual interest rate for funds borrowed, under the debt agreement.

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

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Weighted average interest rate of debt outstanding.

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Fixed interest rate related to the interest rate derivative.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of all notes and loans payable (with maturities initially due after one year or beyond the operating cycle if longer).

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