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Equity - Additional Information (Details) - USD ($)															3 Months Ended		9 Months Ended
Equity - Additional Information (Details) - USD ($)	Oct. 04, 2018	Oct. 01, 2018	Aug. 02, 2018	Jul. 05, 2018	Jul. 02, 2018	Apr. 17, 2018	Apr. 05, 2018	Apr. 02, 2018	Feb. 26, 2018	Jan. 29, 2018	Jan. 04, 2018	Jan. 02, 2018	Jan. 10, 2017	Jul. 10, 2015	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Class of Stock
Authorized capital shares of common stock (in shares)															500,000,000		500,000,000		500,000,000
Authorized capital shares of preferred stock (in shares)															100,000,000		100,000,000		100,000,000
Common stock, shares issued (in shares)															49,576,222		49,576,222		44,937,763
Common stock, shares outstanding (in shares)															49,576,222		49,576,222		44,937,763
Preferred stock issued (in shares)															0		0		0
Preferred stock outstanding (in shares)															0		0		0
Percentage of Operating Partnership held															74.30%		74.30%		72.00%
Aggregate cash dividends and distributions, paid																	$ 37,550,000	$ 31,740,000
Dividends and distributions declared per common share and unit (in dollars per share)			$ 0.20												$ 0.20	$ 0.19	$ 0.60	$ 0.57
Parkway Centre
Class of Stock
Acquisition, common units/shares issued or to be issued (in shares)										117,228
Consolidated Entities Under Development Or Construction \| Operating Partnership
Class of Stock
Ownership interest percentage in properties															$ 0		$ 0		$ 0
Class A units
Class of Stock
Class A Units not held by Company (in shares)															17,166,899		17,166,899
Units redeemed (in shares)					123,504			187,142
Payments for redemption of partnership units								$ 2,500,000
Class A units \| Operating Partnership
Class of Stock
Aggregate cash dividends and distributions, paid				$ 3,500,000			$ 3,500,000				$ 3,300,000
Class B units \| Columbus Village
Class of Stock
Acquisition, common units/shares issued or to be issued (in shares)														1,000,000
Class C units \| Columbus Village
Class of Stock
Acquisition, common units/shares issued or to be issued (in shares)													275,000
Common Stock
Class of Stock
Shares issued (in shares)												163,000					4,227,978
Common stock, shares outstanding (in shares)															49,576,222		49,576,222		44,937,763
Units redeemed (in shares)																	(286,504)
Aggregate cash dividends and distributions, paid				$ 9,700,000			$ 9,000,000				$ 8,500,000
Noncontrolling interests \| Class A units \| Johns Hopkins Village
Class of Stock
Shares issued (in shares)						36,684
New ATM Program
Class of Stock
Consideration received on transaction																	$ 59,700,000
New ATM Program \| Common Stock
Class of Stock
Maximum aggregate offering price of shares to be sold (up to)									$ 125,000,000
Shares issued (in shares)																	4,227,978
Weighted average price (in dollars per share)															$ 14.33		$ 14.33
Subsequent Event
Class of Stock
Shares issued (in shares)		52,200
Subsequent Event \| Class A units
Class of Stock
Units redeemed (in shares)		56,495
Payments for redemption of partnership units		$ 100,000
Subsequent Event \| Class A units \| Operating Partnership
Class of Stock
Aggregate cash dividends and distributions, paid	$ 3,400,000
Subsequent Event \| Common Stock
Class of Stock
Aggregate cash dividends and distributions, paid	$ 9,900,000