Issuance of Common Stock (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended			1 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Sep. 30, 2011 Common stock	Mar. 31, 2011 Common stock	Dec. 31, 2010 Common stock	Sep. 30, 2010 Common stock
Issuance of common stock
Shares issued				6,300,000	8,625,000	7,360,000	6,198,500
Issuance price (in dollars per share)				$ 34.00	$ 34.81	$ 33.70	$ 33.40
Underwriting discounts and offering costs	$ 13,773	$ 25,200	$ 22,471	$ 10,600	$ 14,600
Net proceeds from issuance of common shares		489,236	432,591	203,600	285,600	235,700	196,900
Repayment of borrowings under acquisition credit facility	1,153,400	375,400	616,800			179,800	49,700
Amount used to fund property acquisitions	$ 1,015,725	$ 953,175	$ 713,198				$ 126,500