Changes in Outstanding Common Stock (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Capital Unit [Line Items]
Shares outstanding, beginning balance	83,313,677	83,193,310	82,156,400
Common stock issued through conversion of OP Units	435,756	29,566	749,814
Common stock issued through exercise of options	0	20,000	160,000
Stock Issued During Period, Shares, Restricted Stock Award, Gross	186,666	173,332	214,664
Stock Issued During Period, Shares, Restricted Stock Award, Net of Forfeitures			177,998
Common stock issued through ESPP and Dividend Reinvestment Plan	31,203	19,013	31,108
Common stock repurchased and retired	(87,523)	(121,544)	(82,010)
Shares outstanding, ending balance	83,879,779	83,313,677	83,193,310