Earnings per Share and Unit (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Income per common unit - diluted
Weighted Average Limited Partnership Units Outstanding, Basic	154,170,000	154,170
Parent Company [Member]
Earnings Per Share [Abstract]
Income from operations	$ 40,709	$ 30,189
Gain on sale of real estate	12,868	803
Income from operations attributable to the Company	53,143	30,440
Preferred stock dividends	(5,266)	(5,442)
Net income attributable to common stockholders	$ 47,877	$ 25,174
Weighted average common units outstanding for basic EPU (in shares)	97,518,000	93,842,000
Weighted average common units outstanding for diluted EPU (in shares)	97,891,000	93,907,000
Income per common share/unit - basic:
Continuing operations (in dollars per share)	$ 0.49	$ 0.27
Income per common share/unit - diluted:
Continuing operations (in dollars per share)	$ 0.49	$ 0.27
Partnership Interest [Member]
Earnings Per Share [Abstract]
Income from operations	$ 40,709	$ 30,189
Gain on sale of real estate	12,868	803
Income from operations attributable to the Company	53,228	30,489
Less: preferred unit distributions and other	$ 5,266	$ 5,442
Weighted average common units outstanding for basic EPU (in shares)	97,672,000	93,996,000
Weighted average common units outstanding for diluted EPU (in shares)	98,045,000	94,061,000
Income per common unit - basic
Continuing operations (in dollars per share)	$ 0.49	$ 0.27
Income per common unit - diluted
Continuing operations (in dollars per share)	$ 0.49	$ 0.27
Continuing Operations [Member] | Parent Company [Member]
Earnings Per Share [Abstract]
Net income attributable to common stockholders	$ 47,877	$ 24,998
Net income for common stock unit/holders - diluted	47,877	25,026
Continuing Operations [Member] | Partnership Interest [Member]
Earnings Per Share [Abstract]
Net income attributable to common stockholders	47,962	25,047
Net income for common stock unit/holders - diluted	$ 47,962	$ 25,075