Newly Issued Accounting Pronouncements (Details) $ in Thousands		12 Months Ended
	Jan. 01, 2018 USD ($)	Dec. 31, 2018 USD ($) center capital_lease administrative_office	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 01, 2019 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment of new accounting standards	$ 3,956
Leasing deferred costs		$ 9,200	$ 9,500	$ 9,000
Number of properties subject to ground leases | center		12
Operating leases, number of administrative offices | administrative_office		4
Capital leases, number | capital_lease		1
Capital leased assets, number of units | center		2
Obligations under capital leases		$ 21,898	21,000
Real estate taxes, net		69,268	75,636	66,358
Accounting Standards Update 2017-07 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Defined benefit plan, net periodic benefit cost (credit), excluding the service cost component			400	700
Accounting Standards Update 2018-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Tax Cuts and Jobs Act of 2017, reclassification from AOCI to Retained Earnings for stranded income tax	(800)
Minimum [Member] | Scenario, Forecast [Member] | Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, right-of-use asset					$ 40,000
Operating leases, liabilities					40,000
Maximum [Member] | Scenario, Forecast [Member] | Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, right-of-use asset					45,000
Operating leases, liabilities					$ 45,000
Lease Arrangement, Lessor [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Real estate taxes, net		$ 4,300	$ 4,600	$ 4,200
Net Income Less Than Accumulated Dividends
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment of new accounting standards	5,497
Net Income Less Than Accumulated Dividends | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment of new accounting standards	300
Net Income Less Than Accumulated Dividends | Accounting Standards Update 2016-01 And 2018-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment of new accounting standards	1,500
Net Income Less Than Accumulated Dividends | Accounting Standards Update 2017-05 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment of new accounting standards	3,600
Accumulated Other Comprehensive Loss
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment of new accounting standards	(1,541)
Accumulated Other Comprehensive Loss | Accounting Standards Update 2016-01 And 2018-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment of new accounting standards	(1,500)
Accrued Rent, Accrued Contract Receivables, And Accounts Receivable [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment for new accounting standards on net assets	300
Other Assets [Member] | Accounting Standards Update 2017-05 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment for new accounting standards on net assets	$ 3,600