Summary of Significant Accounting Policies - Deferred Charges, Segment Information and Shareholder Activism Costs (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) segment area	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Shareholder activism	$ 19,369
Capitalized internal leasing costs		$ 22,536	$ 22,403	$ 25,788
Segment Information:
Number of business segments | segment		1
Number of geographic areas in which the Company operates | area		1
Minimum
Deferred Charges:
Deferred lease costs, amortization period (in years)		1 year
Deferred financing costs, amortization period (in years)		1 year
Maximum
Deferred Charges:
Deferred lease costs, amortization period (in years)		15 years
Deferred financing costs, amortization period (in years)		15 years