Consolidated Statements of Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Cumulative Distributions	Accumulated Deficit	Accumulated Other Comprehensive Loss	Total Stockholders’ Equity	Non- controlling Interests
BALANCE at Dec. 31, 2014	$ 990,985	$ 1,326	$ 1,128,318	$ (104,429)	$ (51,285)	$ (423)	$ 973,507	$ 17,478
BALANCE (in shares) at Dec. 31, 2014		129,763,016
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares pursuant to Signature Office REIT merger	433,667	$ 42	433,625				433,667
Issuance of shares pursuant to Signature Office REIT merger (in shares)		41,764,968
Adjustment to par value - common stock		$ (1,217)	1,217
Adjustments to redemption value of redeemable noncontrolling interests	(10,473)		(10,473)				(10,473)
Stock-based compensation	12		12				12
Stock-based compensation (in shares)		667
Offering costs	(62)		(62)				(62)
Distributions to common stockholders	(55,045)			(55,045)			(55,045)
Issuance of shares for distribution reinvestment plan		$ 28	52,529	(52,557)
Issuance of shares for distribution reinvestment plan (in shares)		5,053,669
Repurchase of common stock	(13,819)	$ (4)	(13,815)				(13,819)
Repurchase of common stock (in shares)		(1,397,801)
Additions to common stock subject to redemption	(35,232)		(35,232)				(35,232)
Issuance of limited partnership units	7,282		0				0	7,282
Distributions to noncontrolling interests	(3,150)							(3,150)
Distributions to noncontrolling interests subject to redemption	(10)							(10)
Write-off of offering costs on redemption of preferred units	5,380		5,380				5,380
Net income	(3,888)				(3,750)		(3,750)	(138)
Other comprehensive loss	(6,560)					(6,416)	(6,416)	(144)
BALANCE at Dec. 31, 2015	$ 1,309,087	$ 175	1,561,499	(212,031)	(55,035)	(6,839)	1,287,769	21,318
BALANCE (in shares) at Dec. 31, 2015	175,184,519	175,184,519
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	$ 14		14				14
Stock-based compensation (in shares)		1,333
Distributions to common stockholders	(51,919)			(51,919)			(51,919)
Issuance of shares for distribution reinvestment plan	39,252	$ 4	39,248	(39,252)
Issuance of shares for distribution reinvestment plan (in shares)		3,774,254
Repurchase of common stock	(29,933)	$ (3)	(29,930)				(29,933)
Repurchase of common stock (in shares)		(3,014,538)
Additions to common stock subject to redemption	(9,320)		(9,320)				(9,320)
Issuance of limited partnership units	11,941							11,941
Distributions to noncontrolling interests	(3,022)							(3,022)
Distributions to noncontrolling interests subject to redemption	(9)							(9)
Net income	25,830				24,934		24,934	896
Other comprehensive loss	(18,062)					(17,440)	(17,440)	(622)
BALANCE at Sep. 30, 2016	$ 1,234,607	$ 176	$ 1,561,511	$ (303,202)	$ (30,101)	$ (24,279)	$ 1,204,105	$ 30,502
BALANCE (in shares) at Sep. 30, 2016	175,945,568	175,945,568