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CONDENSED CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Shares	Additional Paid In Capital	Cumulative Net Income	Cumulative Other Comprehensive Loss	Cumulative Common Distributions
Balance (in shares) at Dec. 31, 2018		48,082,903
Balance beginning at Dec. 31, 2018	$ 1,778,968	$ 481	$ 2,609,801	$ 146,882	$ 106	$ (978,302)
Increase (Decrease) in Shareholders' Equity
Share grants (in shares)		9,000
Share grants	865		865
Amount reclassified from cumulative other comprehensive (income) loss to net (income) loss	(371)				(371)
Net current period other comprehensive income (loss)	(32)				(32)
Net income (loss)	34,019			34,019
Distributions to common shareholders	(26,445)					(26,445)
Balance (in shares) at Mar. 31, 2019		48,091,903
Balance ending at Mar. 31, 2019	1,787,004	$ 481	2,610,666	180,901	(297)	(1,004,747)
Balance (in shares) at Dec. 31, 2018		48,082,903
Balance beginning at Dec. 31, 2018	1,778,968	$ 481	2,609,801	146,882	106	(978,302)
Increase (Decrease) in Shareholders' Equity
Net current period other comprehensive income (loss)	(196)
Balance (in shares) at Sep. 30, 2019		48,203,332
Balance ending at Sep. 30, 2019	1,666,613	$ 482	2,612,062	112,188	(461)	(1,057,658)
Balance (in shares) at Mar. 31, 2019		48,091,903
Balance beginning at Mar. 31, 2019	1,787,004	$ 481	2,610,666	180,901	(297)	(1,004,747)
Increase (Decrease) in Shareholders' Equity
Share grants (in shares)		24,000
Share grants	971		971
Share repurchases (in shares)		(2,459)
Share repurchases	(67)		(67)
Net current period other comprehensive income (loss)	(198)				(198)
Net income (loss)	(64,774)			(64,774)
Distributions to common shareholders	(26,450)					(26,450)
Balance (in shares) at Jun. 30, 2019		48,113,444
Balance ending at Jun. 30, 2019	1,696,486	$ 481	2,611,570	116,127	(495)	(1,031,197)
Increase (Decrease) in Shareholders' Equity
Share grants (in shares)		103,100
Share grants	889	$ 1	888
Share repurchases (in shares)		(13,212)
Share repurchases	(396)		(396)
Net current period other comprehensive income (loss)	34				34
Net income (loss)	(3,939)			(3,939)
Distributions to common shareholders	(26,461)					(26,461)
Balance (in shares) at Sep. 30, 2019		48,203,332
Balance ending at Sep. 30, 2019	1,666,613	$ 482	2,612,062	112,188	(461)	(1,057,658)
Balance (in shares) at Dec. 31, 2019		48,201,941
Balance beginning at Dec. 31, 2019	1,705,754	$ 482	2,612,425	177,217	(200)	(1,084,170)
Increase (Decrease) in Shareholders' Equity
Share grants	379		379
Share repurchases (in shares)		(1,012)
Share repurchases	(27)		(27)
Net current period other comprehensive income (loss)	(61)				(61)
Net income (loss)	10,840			10,840
Distributions to common shareholders	(26,511)					(26,511)
Balance (in shares) at Mar. 31, 2020		48,200,929
Balance ending at Mar. 31, 2020	1,690,374	$ 482	2,612,777	188,057	(261)	(1,110,681)
Balance (in shares) at Dec. 31, 2019		48,201,941
Balance beginning at Dec. 31, 2019	1,705,754	$ 482	2,612,425	177,217	(200)	(1,084,170)
Increase (Decrease) in Shareholders' Equity
Net current period other comprehensive income (loss)	200
Balance (in shares) at Sep. 30, 2020		48,318,366
Balance ending at Sep. 30, 2020	1,636,672	$ 483	2,614,346	185,559	0	(1,163,716)
Balance (in shares) at Mar. 31, 2020		48,200,929
Balance beginning at Mar. 31, 2020	1,690,374	$ 482	2,612,777	188,057	(261)	(1,110,681)
Increase (Decrease) in Shareholders' Equity
Share grants (in shares)		28,000
Share grants	1,121		1,121
Share repurchases (in shares)		(1,129)
Share repurchases	(30)		(30)
Net current period other comprehensive income (loss)	176				176
Net income (loss)	1,299			1,299
Distributions to common shareholders	(26,510)					(26,510)
Balance (in shares) at Jun. 30, 2020		48,227,800
Balance ending at Jun. 30, 2020	1,666,430	$ 482	2,613,868	189,356	(85)	(1,137,191)
Increase (Decrease) in Shareholders' Equity
Share grants (in shares)		108,600
Share grants	865	$ 1	864
Share repurchases (in shares)		(18,034)
Share repurchases	(386)		(386)
Amount reclassified from cumulative other comprehensive (income) loss to net (income) loss	85				85
Net current period other comprehensive income (loss)	85
Net income (loss)	(3,797)			(3,797)
Distributions to common shareholders	(26,525)					(26,525)
Balance (in shares) at Sep. 30, 2020		48,318,366
Balance ending at Sep. 30, 2020	$ 1,636,672	$ 483	$ 2,614,346	$ 185,559	$ 0	$ (1,163,716)