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, 2019		48,201,941
Beginning balance at Dec. 31, 2019	$ 1,705,754	$ 482	$ 2,612,425	$ 177,217	$ (200)	$ (1,084,170)
Increase (Decrease) in Shareholders' Equity
Share grants (in shares)		0
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
Ending balance 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
Beginning balance 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
Net income (loss)	8,342
Balance (in shares) at Sep. 30, 2020		48,318,366
Ending balance 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
Beginning balance 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
Ending balance 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)
Net current period other comprehensive income (loss)	85				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
Ending balance at Sep. 30, 2020	1,636,672	$ 483	2,614,346	185,559	$ 0	(1,163,716)
Balance (in shares) at Dec. 31, 2020		48,318,366
Beginning balance at Dec. 31, 2020	1,609,392	$ 483	2,615,305	183,895		(1,190,291)
Increase (Decrease) in Shareholders' Equity
Share grants	321		321
Net income (loss)	37,860			37,860
Distributions to common shareholders	(26,575)					(26,575)
Balance (in shares) at Mar. 31, 2021		48,318,366
Ending balance at Mar. 31, 2021	1,620,998	$ 483	2,615,626	221,755		(1,216,866)
Balance (in shares) at Dec. 31, 2020		48,318,366
Beginning balance at Dec. 31, 2020	$ 1,609,392	$ 483	2,615,305	183,895		(1,190,291)
Increase (Decrease) in Shareholders' Equity
Share repurchases (in shares)	(37,542)
Net current period other comprehensive income (loss)	$ 0
Net income (loss)	(25,125)
Balance (in shares) at Sep. 30, 2021		48,425,924
Ending balance at Sep. 30, 2021	1,505,980	$ 484	2,616,751	158,770		(1,270,025)
Balance (in shares) at Mar. 31, 2021		48,318,366
Beginning balance at Mar. 31, 2021	1,620,998	$ 483	2,615,626	221,755		(1,216,866)
Increase (Decrease) in Shareholders' Equity
Share grants (in shares)		28,000
Share grants	1,176		1,176
Share repurchases (in shares)		(12,009)
Share repurchases	(352)		(352)
Net income (loss)	(66,697)			(66,697)
Distributions to common shareholders	(26,575)					(26,575)
Balance (in shares) at Jun. 30, 2021		48,334,357
Ending balance at Jun. 30, 2021	1,528,550	$ 483	2,616,450	155,058		(1,243,441)
Increase (Decrease) in Shareholders' Equity
Share grants (in shares)		117,800
Share grants	$ 951	$ 1	950
Share repurchases (in shares)	(25,533)	(26,233)
Share repurchases	$ (649)		(649)
Net current period other comprehensive income (loss)	0
Net income (loss)	3,712			3,712
Distributions to common shareholders	(26,584)					(26,584)
Balance (in shares) at Sep. 30, 2021		48,425,924
Ending balance at Sep. 30, 2021	$ 1,505,980	$ 484	$ 2,616,751	$ 158,770		$ (1,270,025)