Earnings per Share and Unit (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Document Fiscal Year Focus			2015
Document Period End Date			Jun. 30, 2015
Earnings Per Share [Abstract]
Gain on sale of real estate	$ 5,657	$ 1,691	$ 6,460	$ 2,406
Income per common unit - diluted
Weighted Average Limited Partnership Units Outstanding, Basic	154,170,000	159,338	154,170	159,804
Parent Company [Member]
Earnings Per Share [Abstract]
Income from operations	$ 32,623	$ 29,470	$ 62,811	$ 53,811
Gain on sale of real estate	5,657	1,691	6,460	2,406
Less: income attributable to noncontrolling interests	534	413	1,087	814
Income from operations attributable to the Company	37,746	30,748	68,184	55,403
Preferred stock dividends	$ (5,266)	$ (5,442)	$ (10,531)	$ (10,884)
Weighted average common units outstanding for basic EPU (in shares)	94,136,000	91,975,000	94,036,000	91,958,000
Weighted average common units outstanding for diluted EPU (in shares)	94,504,000	92,021,000	94,392,000	91,991,000
Income per common share/unit - basic:
Continuing operations (in dollars per share)	$ 0.35	$ 0.28	$ 0.61	$ 0.48
Income per common share/unit - diluted:
Continuing operations (in dollars per share)	$ 0.34	$ 0.28	$ 0.61	$ 0.48
Partnership Interest [Member]
Earnings Per Share [Abstract]
Income from operations	$ 32,623	$ 29,470	$ 62,811	$ 53,811
Gain on sale of real estate	5,657	1,691	6,460	2,406
Income from operations attributable to the Company	37,807	30,801	68,294	55,498
Less: preferred unit distributions and other	$ 5,266	$ 5,442	$ 10,531	$ 10,884
Weighted average common units outstanding for basic EPU (in shares)	94,290,000	92,134,000	94,190,000	92,118,000
Weighted average common units outstanding for diluted EPU (in shares)	94,658,000	92,180,000	94,546,000	92,151,000
Income per common unit - basic
Continuing operations (in dollars per share)	$ 0.35	$ 0.28	$ 0.61	$ 0.48
Income per common unit - diluted
Continuing operations (in dollars per share)	$ 0.34	$ 0.28	$ 0.61	$ 0.48
Continuing Operations [Member] | Parent Company [Member]
Earnings Per Share [Abstract]
Net income attributable to common stockholders	$ 32,480	$ 25,306	$ 57,653	$ 44,519
Net income for common stock unit/holders - diluted	32,480	25,328	57,653	44,550
Continuing Operations [Member] | Partnership Interest [Member]
Earnings Per Share [Abstract]
Less: income attributable to noncontrolling interests	473	360	977	719
Net income attributable to common stockholders	32,541	25,359	57,763	44,614
Net income for common stock unit/holders - diluted	$ 32,541	$ 25,381	$ 57,763	$ 44,645