Significant Accounting Policies - Deferred Leasing Commissions and Revenue Recognition (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020 USD ($) shares	Sep. 30, 2020 shares	Jun. 30, 2020 shares	Mar. 31, 2020 shares	Dec. 31, 2019 shares	Sep. 30, 2019 shares	Jun. 30, 2019 shares	Mar. 31, 2019 shares	Dec. 31, 2020 USD ($) item shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares
Deferred Leasing Commissions
Amortization of deferred leasing commissions											$ 8,500
Amortization of deferred leasing commissions									$ 9,600	$ 9,900
Estimated annual amortization of deferred leasing commissions for the succeeding five years
2021									9,957
2022									9,167
2023									8,041
2024									6,797
2025									5,621
2026 and thereafter									16,869
Summary of rental revenue
Income from leases									180,899	191,828	202,127
Reimbursable expenses									61,310	64,421	61,475
Straight Line Rent Adjustments									1,685	8,876	(381)
Amortization of favorable and unfavorable leases									313	402	556
Total											$ 263,777
Total									$ 244,207	$ 265,527
Segment Reporting
Number of reporting segments | item									1
Net Income Per Share
Potential dilutive shares outstanding | shares									0	0	0
Denominator used for calculating basic and diluted net income per share (in shares) | shares	107,328,000	107,328,000	107,287,000	107,269,000	107,240,000	107,231,000	107,231,000	107,231,000	107,303,000	107,233,000	107,231,000
Derivative instruments
Fair value hedges outstanding	$ 0								$ 0
Rental
Summary of rental revenue
Total											$ 263,777
Total									$ 244,207	$ 265,527