Quarterly Supplemental Financial Information (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 Supplemental Financial Information [Abstract]
Total income	$ 54,740	$ 58,169	$ 57,190	$ 56,391	$ 57,256	$ 60,094	$ 61,490	$ 63,834	$ 226,490	$ 242,674	$ 251,809
Net income from continuing operations	28,694	24,926	6,278	4,001					63,899	83,849	57,954
Net (loss) income from discontinued operations	0	0	(12,000)	(13,500)					(25,500)	0	3,839
Net income (loss)	$ 28,694	$ 24,926	$ (5,722)	$ (9,499)	$ 17,507	$ 8,947	$ 23,163	$ 34,232	$ 38,399	$ 83,849	$ 61,793
Net loss per common share, from discontinued operations, basic and diluted (in dollars per share)	$ 0	$ 0	$ (0.02)	$ (0.02)					$ (0.04)	$ 0	$ 0
Net income (loss), per common share, basic and diluted (in dollars per share)	$ 0.04	$ 0.03	$ (0.01)	$ (0.01)	$ 0.03	$ 0.01	$ 0.03	$ 0.04	$ 0.05	$ 0.11	$ 0.07
Weighted average number of common shares outstanding, basic	728,544,149	728,722,763	728,654,374	728,558,989	727,904,818	768,385,770	774,391,881	774,311,254	728,620,309	761,139,011	773,445,341
Weighted average number of common shares outstanding, diluted	729,536,570	729,456,722	729,287,663	728,827,861	728,973,047	769,306,859	774,930,240	774,394,732	729,384,149	762,065,474	774,600,479