Stockholder's Equity (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Class of Stock [Line Items]
Gross proceeds from the issuance of common stock	$ 225,000,000
Stock issued during period	6,122,000	5,750,000
Sale of stock, price per share	$ 36.75	$ 31.50
Proceeds from sale of common stock, net of issuance costs	$ 219,084,000	$ 175,284,000