Equity - Additional Information (Details)																		12 Months Ended
	Jan. 03, 2019 USD ($)	Jan. 02, 2019 shares	Oct. 01, 2018 USD ($) shares	Jul. 02, 2018 shares	Apr. 17, 2018 $ / shares shares	Apr. 02, 2018 USD ($) shares	Feb. 26, 2018 USD ($)	Jan. 29, 2018 shares	Jan. 02, 2018 shares	Dec. 21, 2017 USD ($)	Oct. 02, 2017 USD ($) shares	May 12, 2017 USD ($) $ / shares shares	Jan. 10, 2017 shares	Oct. 13, 2016 USD ($) shares	May 04, 2016 USD ($)	Jul. 10, 2015 shares	May 05, 2015 USD ($)	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 20, 2017 USD ($)	Dec. 31, 2015 shares
Class of Stock [Line Items]
Authorized capital shares of common stock (in shares)																		500,000,000	500,000,000
Preferred stock, shares authorized (in shares)																		100,000,000	100,000,000
Common stock, shares issued (in shares)																		50,013,731	44,937,763
Common stock, shares outstanding (in shares)																		50,013,731	44,937,763
Preferred stock, shares issued (in shares)																		0	0
Preferred stock, shares outstanding (in shares)																		0	0
Shares issued through public offering (in shares)			52,200
Acquisition, common units/shares issued (in shares)														2,000,000
Acquisition, contract price | $														$ 26,200,000
Redeemable noncontrolling interest, equity percentage																					20.00%
Redeemable noncontrolling interest | $										$ 2,000,000
Noncontrolling interest change in redemption value | $										$ 500,000
Percentage of operating partnerships held																		74.50%	72.00%
Operating partnership units redemption ratio																		1
Dividend Per Share/Distribution Per Unit (in dollars per share) | $ / shares																		$ 0.80	$ 0.76	$ 0.72
Aggregate cash dividends and distributions, paid | $																		$ 50,897,000	$ 43,616,000	$ 33,843,000
Johns Hopkins Village
Class of Stock [Line Items]
Redeemable noncontrolling interest carrying amount | $																					$ 2,000,000
Town Center Phase VI
Class of Stock [Line Items]
Noncontrolling interest ownership percentage in properties													20.00%
Class A units
Class of Stock [Line Items]
Units not held by company																		17,110,404
Units redeemed (in shares)			56,495	123,504		187,142					358,879
Payments for redemption of partnership units | $			$ 100,000			$ 2,500,000					$ 4,900,000
Class A units | Town Center Phase VI
Class of Stock [Line Items]
Acquisition, common units/shares issued (in shares)													68,691
Class A units | Operating Partnership
Class of Stock [Line Items]
Aggregate cash dividends and distributions, paid | $																		$ 13,800,000	$ 12,600,000	$ 11,100,000
Class A units | Operating Partnership | Subsequent Event
Class of Stock [Line Items]
Aggregate cash dividends and distributions, paid | $	$ 3,400,000
Common stock
Class of Stock [Line Items]
Common stock, shares outstanding (in shares)																		50,013,731	44,937,763	37,490,361		30,076,359
Shares issued through public offering (in shares)									163,000									4,617,409	7,350,690	5,312,855
Units redeemed (in shares)																		338,704
Aggregate cash dividends and distributions, paid | $																		$ 38,700,000	$ 31,100,000	$ 22,700,000
Common stock | Subsequent Event
Class of Stock [Line Items]
Shares issued through public offering (in shares)		118,471
Aggregate cash dividends and distributions, paid | $	$ 10,000,000
Columbus Village | Class B units
Class of Stock [Line Items]
Acquisition, common units/shares issued (in shares)																1,000,000
Columbus Village | Class C units
Class of Stock [Line Items]
Acquisition, common units/shares issued (in shares)													275,000
Parkway Centre | Class A units
Class of Stock [Line Items]
Acquisition, common units/shares issued (in shares)								117,228
2015 ATM Program
Class of Stock [Line Items]
Maximum aggregate offering price of shares to be sold (up to) | $																	$ 50,000,000.0
Shares issued through public offering (in shares)																				1,152,919
Net proceeds after offering costs and commissions from sale of shares | $																				$ 12,200,000
2016 ATM Program | Common stock
Class of Stock [Line Items]
Maximum aggregate offering price of shares to be sold (up to) | $															$ 75,000,000.0
Shares issued through public offering (in shares)																			450,890	4,159,936
Net proceeds after offering costs and commissions from sale of shares | $																			$ 6,200,000	$ 54,800,000
2018 ATM Program | Common stock
Class of Stock [Line Items]
Maximum aggregate offering price of shares to be sold (up to) | $							$ 125,000,000
Shares issued through public offering (in shares)																		4,617,409
Net proceeds after offering costs and commissions from sale of shares | $																		$ 65,200,000
Underwritten Public Offering | Common stock
Class of Stock [Line Items]
Shares issued through public offering (in shares)												6,900,000.0
Net proceeds after offering costs and commissions from sale of shares | $												$ 85,300,000
Issuance of common stock, price per share (in dollars per share) | $ / shares												$ 13.00
Johns Hopkins Village | Noncontrolling interests | Class A units
Class of Stock [Line Items]
Shares issued through public offering (in shares)					36,684
Share price (in dollars per share) | $ / shares					$ 13.77
Weighted Average | 2015 ATM Program
Class of Stock [Line Items]
Issuance of common stock, price per share (in dollars per share) | $ / shares																				$ 10.87
Weighted Average | 2016 ATM Program | Common stock
Class of Stock [Line Items]
Issuance of common stock, price per share (in dollars per share) | $ / shares																			$ 14.08	$ 13.45
Weighted Average | 2018 ATM Program | Common stock
Class of Stock [Line Items]
Issuance of common stock, price per share (in dollars per share) | $ / shares																		$ 14.39