Debt (Schedule of Debt) (Details)	12 Months Ended
Dec. 31, 2019 USD ($) extension	Dec. 31, 2018 USD ($)
Debt Instrument [Line Items]
Subtotal/Weighted Average	3.76%	3.76%
Amount Outstanding	$ 1,481,404,000	$ 1,685,472,000
Seven Year Interest Rate Swap Agreement \| Interest rate swaps
Debt Instrument [Line Items]
Derivative, notional amount	$ 100,000,000
Secured Debt
Debt Instrument [Line Items]
Subtotal/Weighted Average	3.79%
Amount Outstanding	$ 189,030,000	190,351,000
Premium (discount) and unamortized debt issuance costs	$ 343,000	645,000
Secured Debt \| Secured 35 Million Mortgage Loan
Debt Instrument [Line Items]
Stated Rate	5.55%
Effective Rate	3.75%
Amount Outstanding	$ 28,687,000	29,706,000
Face amount of facility	$ 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	3.76%
Amount Outstanding	$ 1,292,374,000	1,495,121,000
Premium (discount) and unamortized debt issuance costs	(7,626,000)	(9,879,000)
Derivative, notional amount	$ 450,000,000
Unsecured Debt \| Amended and Restated $300 Million Unsecured 2011 Term Loan
Debt Instrument [Line Items]
Effective Rate	3.20%
Amount Outstanding	$ 300,000,000	300,000,000.0
Face amount of facility	$ 300,000,000
Unsecured Debt \| Amended and Restated $300 Million Unsecured 2011 Term Loan \| LIBOR
Debt Instrument [Line Items]
Spread on variable rate	1.00%
Unsecured Debt \| Unsecured 500 Million 2018 Line Of Credit
Debt Instrument [Line Items]
Face amount of facility	$ 500,000,000
Unsecured Debt \| Unsecured 500 Million 2018 Line Of Credit \| Line of Credit
Debt Instrument [Line Items]
Effective Rate	2.70%
Amount Outstanding	$ 0	205,000,000
Additional extension period	1 year
Number of extension periods \| extension	2
Maturity extension period	6 months
Unsecured Debt \| Unsecured 500 Million 2018 Line Of Credit \| LIBOR
Debt Instrument [Line Items]
Spread on variable rate	0.90%
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	3.83%
Amount Outstanding	$ 250,000,000	$ 250,000,000
Face amount of facility	250,000,000
Derivative, notional amount	$ 150,000,000
Unsecured Debt \| $250 Million Unsecured 2018 Term Loan \| Through March 29, 2020
Debt Instrument [Line Items]
Derivative fixed interest rate	4.11%
Unsecured Debt \| $250 Million Unsecured 2018 Term Loan \| From March 30, 2020 Through March 31, 2025
Debt Instrument [Line Items]
Derivative fixed interest rate	4.21%
Unsecured Debt \| $250 Million Unsecured 2018 Term Loan \| LIBOR
Debt Instrument [Line Items]
Spread on variable rate	1.60%

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

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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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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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