Property Transactions Property Transactions (Acquisition Pro Forma) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Revenues	$ 41,268	$ 34,652
Income (Loss) from Continuing Operations, Including Portion Attributable to Noncontrolling Interest	56,895	(1,691)
Net income	56,904	(11,354)
Net Income (Loss) Available to Common Stockholders, Basic	53,170	(13,112)
Post Oak Central and Terminus Pro Forma [Member]
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Revenues	42,479	37,001
Income (Loss) from Continuing Operations, Including Portion Attributable to Noncontrolling Interest	57,585	(1,072)
Net income	57,594	(10,735)
Net Income (Loss) Available to Common Stockholders, Basic	$ 53,860	$ (12,493)
Earnings Per Share, Basic	$ 0.52	$ (0.12)
Earnings Per Share, Diluted	$ 0.52	$ (0.12)