Quarterly Operating Results (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 179,629	$ 201,793	$ 205,717	$ 182,178	$ 198,941	$ 208,963	$ 212,272	$ 196,245	$ 769,317	$ 816,421	$ 770,864
Net income (Loss)	11,932	30,571	43,670	14,089	18,422	(35,535)	74,438	16,637	100,262	73,962	94,995
Net Income (Loss) Attributable to the Company	11,899	30,443	43,512	14,034	18,358	(35,423)	74,190	16,579	99,888	73,704	94,668
Net income (loss) Attributable to common shareholders	$ 7,875	$ 26,420	$ 39,488	$ 10,011	$ 14,334	$ (43,897)	$ 69,949	$ 6,566	$ 83,794	$ 46,952	$ 68,718
Net income (loss) per share available to common shareholders, basic (usd per share)	$ 0.11	$ 0.38	$ 0.57	$ 0.14	$ 0.20	$ (0.61)	$ 0.97	$ 0.09	$ 1.20	$ 0.65	$ 0.95
Net income (loss) per share available to common shareholders, diluted (in usd per share)	$ 0.11	$ 0.38	$ 0.57	$ 0.14	$ 0.20	$ (0.61)	$ 0.96	$ 0.09	$ 1.19	$ 0.64	$ 0.94