Issuance of Common Stock (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Issuance of Common Stock [Abstract]
Net proceeds from issuance of common shares	$ 0	$ 285,533,000
Common Stock [Member]
Shares of common stock issued [Abstract]
Issued shares (in shares)		8,625,000
Issuance of Common Stock [Abstract]
Common stock issuance price (in dollars per share)		$ 34.81
Offering costs of issuance of common stock		14,700,000
Net proceeds from issuance of common shares		$ 285,533,000