Earnings Per Share (EPS) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Earnings Per Share [Abstract]
Net income attributable to common stockholders	$ 13,363		$ 13,635		$ 26,339		$ 25,717
Add back: Net income attributable to noncontrolling interests in our Operating Partnership	2,592		2,711		5,171		5,127
Numerator for diluted net income attributable to all equity holders	$ 15,955		$ 16,346		$ 31,510		$ 30,844
Weighted average shares of common stock outstanding - basic	143,717		142,581		143,426		142,511
Operating partnership units and vested long term incentive plan (LTIP) units	27,863	[1]	28,341	[1]	28,155	[1]	28,412	[1]
Stock options	4,228	[1]	3,736	[1]	4,003	[1]	3,461	[1]
Unvested LTIP units	502	[1]	594	[1]	469	[1]	553	[1]
Weighted average shares of common stock and common stock equivalents outstanding - diluted	176,310	[1]	175,252	[1]	176,053	[1]	174,937	[1]
Net income attributable to common stockholders per share – basic (usd per share)	$ 0.09		$ 0.10		$ 0.18		$ 0.18
Net income attributable to common stockholders per share – diluted (usd per share)	$ 0.09		$ 0.09		$ 0.18		$ 0.18

[1] Diluted shares are calculated in accordance with GAAP, and represent ownership in our company through shares of common stock, units in our operating partnership and other convertible equity instruments.