Quarterly Financial Information (unaudited) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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]
Revenues	$ 120,817	$ 119,717	$ 118,449	$ 122,703	$ 119,354	$ 119,137	$ 119,164	$ 124,842	$ 481,686	$ 482,497	$ 538,139
Net income (loss)	16,172	(28,153)	21,170	23,208	12,144	12,834	10,882	41,780	32,397	77,640	251,491
Net income (loss) attributable to common shareholders	$ 16,172	$ (28,153)	$ 21,170	$ 23,208	$ 12,144	$ 12,834	$ 10,882	$ 41,780	$ 32,397	$ 77,640	$ 237,624
Net income (loss) per common share attributable to common shareholders – basic and diluted (in dollars per share)	$ 0.08	$ (0.13)	$ 0.10	$ 0.11	$ 0.06	$ 0.06	$ 0.05	$ 0.19	$ 0.15	$ 0.35	$ 1.03
Weighted average number of common shares outstanding – basic	212,996	212,995	212,951	212,850	214,684	218,808	218,982	218,849	212,948	217,830	230,747
Weighted average number of common shares outstanding – diluted	213,627	212,995	213,090	213,223	215,093	219,021	219,410	219,403	213,198	218,231	230,927