Significant Transactions During the First Quarter of 2013 and Subsequent Events (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Jan. 31, 2013 Annaliese [Member] Unit	Feb. 28, 2013 Fox Plaza [Member] Unit
Acquisitions [Abstract]
Cost of acquired entity			$ 19,000,000	$ 135,000,000
Number of units			56	444
Common stock [Abstract]
Number of common shares sold (in shares)	817,445
Proceeds from sale of common shares	122,905,000
Average price per share of common shares sold (dollars per share)	$ 151.82
Unsecured Bond Offering [Abstract]
Aggregate principal amount of debt issued	300,000,000
Coupon Rate (in hundredths)	3.25%
Net proceeds from issuance of notes	295,200,000
Dispositions [Abstract]
Sale of land held for future development	9,100,000
Gain on sale of land	$ 1,503,000	$ 0