CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Series D	Series E	Cumulative Preferred Distributions	Common Shares	Cumulative Common Distributions	Additional Paid in Capital	Cumulative Net Income	Cumulative Other Comprehensive Loss	Noncontrolling Interest
Balance (in shares) at Dec. 31, 2015		4,915,196	11,000,000		126,349,914
Balance at Dec. 31, 2015	$ 3,368,487	$ 119,263	$ 265,391	$ (650,195)	$ 1,263	$ (3,111,868)	$ 4,414,611	$ 2,333,709	$ (3,687)	$ 0
Comprehensive income:
Net income	232,894							232,894		0
Unrealized gain (loss) on derivative instrument	3,479								3,479
Unrealized gain on marketable securities	0
Total comprehensive income	236,373
Repurchase of shares (in shares)					(2,491,675)
Repurchase of shares	(69,987)				$ (25)		(69,962)			0
Redemption of shares (in shares)		0	(11,000,000)		0
Redemption of shares	(275,000)	$ 0	$ (275,000)	0	$ 0		0
Excess fair value of consideration paid over carrying value of preferred shares	(9,609)		$ 9,609	(9,609)
Share-based compensation (in shares)					136,226
Share-based compensation	18,530				$ 2		18,528			0
Distributions	(17,956)			(17,956)		0				0
Balance (in shares) at Dec. 31, 2016		4,915,196	0		123,994,465
Balance at Dec. 31, 2016	3,260,447	$ 119,263	$ 0	(677,760)	$ 1,240	(3,111,868)	4,363,177	2,566,603	(208)	0
Comprehensive income:
Net income	29,666							29,656		10
Unrealized gain (loss) on derivative instrument	(248)								(248)
Unrealized gain on marketable securities	361								361	0
Total comprehensive income	29,779
Repurchase of shares (in shares)					(43,329)
Repurchase of shares	(3,188)				$ 0		(3,188)			0
Excess fair value of consideration paid over carrying value of preferred shares	0
Share-based compensation (in shares)					266,480
Share-based compensation	21,414				$ 2		20,156			1,256
Contributions	31									31
Distributions	(7,988)			(7,988)		0				0
Adjustment for noncontrolling interest	$ 0						168			(168)
Balance (in shares) at Dec. 31, 2017	124,217,616	4,915,196	0		124,217,616
Balance at Dec. 31, 2017	$ 3,300,495	$ 119,263	$ 0	(685,748)	$ 1,242	(3,111,868)	4,380,313	2,596,259	(95)	1,129
Comprehensive income:
Reclassification pursuant to change in accounting principle	0
Reclassification pursuant to change in accounting principle | Accounting Standards Update 2016-01								1,902	(1,902)
Net income	272,908							272,813		95
Unrealized gain (loss) on derivative instrument	456								456
Unrealized gain on marketable securities	1,199								1,199
Total comprehensive income	274,563
Repurchase of shares (in shares)					(3,163,730)
Repurchase of shares	(93,976)				$ (31)		(93,945)
Excess fair value of consideration paid over carrying value of preferred shares	0
Share-based compensation (in shares)					518,269
Share-based compensation	19,697				$ 5		18,371			1,321
Contributions	1									1
Distributions	(316,782)			(7,988)		(308,680)				(114)
Adjustment for noncontrolling interest	$ 0						1,235			(1,235)
Balance (in shares) at Dec. 31, 2018	121,572,155	4,915,196	0		121,572,155
Balance at Dec. 31, 2018	$ 3,183,998	$ 119,263	$ 0	$ (693,736)	$ 1,216	$ (3,420,548)	$ 4,305,974	$ 2,870,974	$ (342)	$ 1,197