Significant Accounting Policies - Deferred Leasing Commissions and Revenue Recognition (Details)	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($) shares	Sep. 30, 2017 shares	Jun. 30, 2017 shares	Mar. 31, 2017 shares	Dec. 31, 2016 shares	Sep. 30, 2016 shares	Jun. 30, 2016 shares	Mar. 31, 2016 shares	Dec. 31, 2017 USD ($) item shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares
Deferred Leasing Commissions
Amortization of deferred leasing commissions									$ 6,919,000	$ 6,272,000	$ 5,680,000
Estimated annual amortization of deferred leasing commissions for the succeeding five years
2018									7,655,000
2019									7,027,000
2020									6,148,000
2021									5,379,000
2022									4,112,000
2023 and thereafter									10,358,000
Summary of rental revenue
Income from leases									205,690,000	192,055,000	185,738,000
Reimbursable expenses									58,777,000	49,821,000	49,512,000
Straight-line rent adjustment									1,767,000	1,977,000	2,448,000
Amortization of favorable and unfavorable leases									1,031,000	496,000	158,000
Rental revenue									$ 267,265,000	$ 244,349,000	$ 237,856,000
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	107,231,000	107,231,000	107,231,000	107,231,000	107,231,000	103,709,000	100,187,000	100,187,000	107,231,000	102,843,000	100,187,000
Derivative instruments
Fair value hedges outstanding	$ 0								$ 0
ASU 2016-02
Recent Accounting Standards
Increase in assets and liabilities									$ 3,000,000