Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 87,251	$ 99,958	$ 105,493	$ 102,637	$ 102,169	$ 97,689	$ 95,684	$ 96,099	$ 395,339	$ 391,641	$ 429,496
Net income	25,674	220,850	(795)	(14,823)	31,448	5,596	7,365	42,220	230,906	86,629	96,450
Net income (loss) attributable to common shareholders	$ 23,796	$ 216,190	$ (3,327)	$ (15,957)	$ 29,235	$ 3,916	$ 5,519	$ 40,397	$ 227,415	$ 85,583	$ 95,624
Net income attributable to common shareholders - basic (usd per share)	$ 0.10	$ 0.91	$ (0.01)	$ (0.07)	$ 0.12	$ 0.02	$ 0.02	$ 0.17	$ 0.93	$ 0.33	$ 0.38
Net income (loss) attributable to common shareholders - per common share diluted (usd per share)	$ 0.10	$ 0.90	$ (0.01)	$ (0.07)	$ 0.12	$ 0.02	$ 0.02	$ 0.17	$ 0.93	$ 0.33	$ 0.37