Selected Quarterly Financial Data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 256,039	$ 255,827	$ 262,003	$ 266,286	$ 285,222	$ 278,969	$ 277,202	$ 275,770	$ 1,040,155	$ 1,117,164	$ 1,074,729
Net income (loss) attributable to common shareholders	$ (51,697)	$ (29,390)	$ (37,229)	$ 30,082	$ (118,543)	$ 45,805	$ 123,587	$ 236,022	$ (88,234)	$ 286,872	$ 147,610
Per share data (basic and diluted):
Net income (loss) attributable to common shareholders (in dollars per share)	$ (0.22)	$ (0.12)	$ (0.16)	$ 0.13	$ (0.50)	$ 0.19	$ 0.52	$ 0.99	$ (0.37)	$ 1.21	$ 0.62
Common distributions declared (in dollars per share)	$ 0.15	$ 0.15	$ 0.15	$ 0.39	$ 0.39	$ 0.39	$ 0.39	$ 0.39