Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Numerator:
Income (loss) from continuing operations	$ 10,882	$ (1,865)	$ 18,143	$ (52,129)
Gain on sales of investment properties	0	116,628	34,519	157,792
Preferred stock dividends	0	(2,363)	0	(4,725)
Net income attributable to common shareholders	10,882	112,400	52,662	100,938
Earnings allocated to unvested restricted shares	(90)	(88)	(172)	(178)
Net income attributable to common shareholders excluding amounts attributable to unvested restricted shares	$ 10,792	$ 112,312	$ 52,490	$ 100,760
Denominator for earnings per common share – basic:
Weighted average number of common shares outstanding	218,982	234,243	218,915	235,269
Effect of dilutive securities:
Stock options	0	1	0	1
RSUs	428	574	491	572
Denominator for earnings per common share – diluted:
Weighted average number of common and common equivalent shares outstanding	219,410	234,818	219,406	235,842
Earnings Per Share, Other Disclosures
Weighted average number of shares of restricted common stock	529	536	551	550
Antidilutive Securities Excluded from Computation of Earnings per Share [Line Items]
Number of outstanding options to purchase shares of common stock	38	41	38	41
Weighted average exercise price of outstanding options (in dollars per share)	$ 18.85	$ 19.25	$ 18.85	$ 19.25
Restricted shares
Earnings Per Share, Other Disclosures
Unvested restricted common stock	521	556	521	556	496
Stock options
Antidilutive Securities Excluded from Computation of Earnings per Share [Line Items]
Number of outstanding options to purchase shares of common stock that would be anti-dilutive			32	35
Weighted average exercise price of outstanding options excluded from diluted EPS calculation (in dollars per share)			$ 20.19	$ 20.55
RSUs
Earnings Per Share, Other Disclosures
Unvested restricted common stock	649		649		555
Antidilutive Securities Excluded from Computation of Earnings per Share [Line Items]
Number of RSUs eligible for future conversion	649	644	649	644
Weighted average number of RSUs	649	644	667	641