Issuance of Common Stock (Details) (USD $)	1 Months Ended		9 Months Ended
	Sep. 30, 2011	Mar. 31, 2011	Sep. 30, 2011	Sep. 30, 2010
Issuance of Common Stock [Abstract]
Issued shares of common stock (in shares)	6,300,000	8,625,000
Issued shares price (in dollars per share)	$ 34.00	$ 34.81
Offering costs of issuance of common stock	$ 10,600,000	$ 14,600,000
Net proceeds from issuance of common shares	$ 203,600,000	$ 285,600,000	$ 489,236,000	$ 196,899,000