Organization and Principles of Consolidation Revenues and Tenant and Other Receivables (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($) unit	Mar. 31, 2018 USD ($)	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Schedule of management, transaction, and other fees [Line Items]
Number of Properties Subject to Ground Leases | unit	22
Last Ground Lease Expiration Date	2101
Last Office Lease Expiration Date	2029
Operating Lease, Liability			$ 225,400
Operating Lease, Right-of-Use Asset			$ 297,800
Management, transaction, and other fee [Member]
Schedule of management, transaction, and other fees [Line Items]
Accounts Receivable, Net	$ 11,000
Parent Company [Member]
Schedule of management, transaction, and other fees [Line Items]
Other Operating Income	1,982	$ 2,025
Property Management Fee Revenue	6,972	7,158
Accounts Receivable, Net	160,635			$ 172,359
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets					$ 30,903
Operating Lease, Liability	225,122			0
Operating Lease, Right-of-Use Asset	296,859			0
Parent Company [Member] | Property management services [Member]
Schedule of management, transaction, and other fees [Line Items]
Other Operating Income	1,982	2,025
Property Management Fee Revenue	3,764	3,768
Parent Company [Member] | Asset management services [Member]
Schedule of management, transaction, and other fees [Line Items]
Property Management Fee Revenue	1,777	1,703
Parent Company [Member] | Leasing services [Member]
Schedule of management, transaction, and other fees [Line Items]
Property Management Fee Revenue	758	685
Parent Company [Member] | Other transaction fees [Member]
Schedule of management, transaction, and other fees [Line Items]
Property Management Fee Revenue	$ 673	$ 1,002
Parent Company [Member] | Management, transaction, and other fee [Member]
Schedule of management, transaction, and other fees [Line Items]
Accounts Receivable, Net				$ 12,500