CONDENSED CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (unaudited) - USD ($) $ in Thousands	Total	Common Shares	Cumulative Common Distributions	Additional Paid in Capital	Cumulative Net Income Available for Common Shareholders	Cumulative Other Comprehensive Income (Loss)
Beginning balance at Dec. 31, 2017	$ 2,755,422	$ 1,643	$ (4,834,491)	$ 4,542,307	$ 2,966,605	$ 79,358
Beginning balance (in shares) at Dec. 31, 2017		164,349,141
Increase (Decrease) in Shareholders' Equity
Net income	80,206				80,206
Equity interest in investee’s unrealized gains (losses)	(93)					(93)
Common share repurchases	(101)			(101)
Common share repurchases (in shares)		(3,394)
Distributions	(85,460)		(85,460)
Ending balance at Mar. 31, 2018	2,749,974	$ 1,643	(4,919,951)	4,542,206	3,126,367	(291)
Ending balance (in shares) at Mar. 31, 2018		164,345,747
Beginning balance at Dec. 31, 2017	2,755,422	$ 1,643	(4,834,491)	4,542,307	2,966,605	79,358
Beginning balance (in shares) at Dec. 31, 2017		164,349,141
Increase (Decrease) in Shareholders' Equity
Net income	294,594
Ending balance at Sep. 30, 2018	2,792,571	$ 1,644	(5,094,169)	4,544,449	3,340,755	(108)
Ending balance (in shares) at Sep. 30, 2018		164,442,379
Beginning balance at Mar. 31, 2018	2,749,974	$ 1,643	(4,919,951)	4,542,206	3,126,367	(291)
Beginning balance (in shares) at Mar. 31, 2018		164,345,747
Increase (Decrease) in Shareholders' Equity
Net income	97,289				97,289
Equity interest in investee’s unrealized gains (losses)	10					10
Common share grants	501	$ 1		500
Common share grants (in shares)		18,000
Distributions	(87,105)		(87,105)
Ending balance at Jun. 30, 2018	2,760,669	$ 1,644	(5,007,056)	4,542,706	3,223,656	(281)
Ending balance (in shares) at Jun. 30, 2018		164,363,747
Increase (Decrease) in Shareholders' Equity
Net income	117,099				117,099
Equity interest in investee’s unrealized gains (losses)	173					173
Common share grants	2,243	$ 1		2,242
Common share grants (in shares)		97,000
Common share repurchases	(500)	$ (1)		(499)
Common share repurchases (in shares)		(18,368)
Distributions	(87,113)		(87,113)
Ending balance at Sep. 30, 2018	2,792,571	$ 1,644	(5,094,169)	4,544,449	3,340,755	(108)
Ending balance (in shares) at Sep. 30, 2018		164,442,379
Beginning balance at Dec. 31, 2018	2,597,431	$ 1,644	(5,181,323)	4,545,481	3,231,895	(266)
Beginning balance (in shares) at Dec. 31, 2018		164,441,709
Increase (Decrease) in Shareholders' Equity
Net income	225,787				225,787
Equity interest in investee’s unrealized gains (losses)	66					66
Common share grants	436			436
Distributions	(87,154)		(87,154)
Ending balance at Mar. 31, 2019	2,736,566	$ 1,644	(5,268,477)	4,545,917	3,457,682	(200)
Ending balance (in shares) at Mar. 31, 2019		164,441,709
Beginning balance at Dec. 31, 2018	2,597,431	$ 1,644	(5,181,323)	4,545,481	3,231,895	(266)
Beginning balance (in shares) at Dec. 31, 2018		164,441,709
Increase (Decrease) in Shareholders' Equity
Net income	274,643
Ending balance at Sep. 30, 2019	2,608,986	$ 1,646	(5,446,078)	4,547,055	3,506,538	(175)
Ending balance (in shares) at Sep. 30, 2019		164,565,303
Beginning balance at Mar. 31, 2019	2,736,566	$ 1,644	(5,268,477)	4,545,917	3,457,682	(200)
Beginning balance (in shares) at Mar. 31, 2019		164,441,709
Increase (Decrease) in Shareholders' Equity
Net income	8,782				8,782
Equity interest in investee’s unrealized gains (losses)	71					71
Common share grants	869	$ 1		868
Common share grants (in shares)		15,000
Common share repurchases and forfeitures	(48)			(48)
Common share repurchases and forfeitures (in shares)		(2,172)
Distributions	(88,798)		(88,798)
Ending balance at Jun. 30, 2019	2,657,442	$ 1,645	(5,357,275)	4,546,737	3,466,464	(129)
Ending balance (in shares) at Jun. 30, 2019		164,454,537
Increase (Decrease) in Shareholders' Equity
Net income	40,074				40,074
Equity interest in investee’s unrealized gains (losses)	(46)					(46)
Common share grants	1,068	$ 1		1,067
Common share grants (in shares)		140,100
Common share repurchases and forfeitures	(749)			(749)
Common share repurchases and forfeitures (in shares)		(29,334)
Distributions	(88,803)		(88,803)
Ending balance at Sep. 30, 2019	$ 2,608,986	$ 1,646	$ (5,446,078)	$ 4,547,055	$ 3,506,538	$ (175)
Ending balance (in shares) at Sep. 30, 2019		164,565,303