Common Stock (Details) (USD $)	1 Months Ended			12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2009	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock Issued During Period, Shares, New Issues	8,050,000	9,200,000		17,288,265	39,872	99,738
Underwriters Over Allotment Of Shares	1,050,000	1,200,000
Common Stock, Dividends, Per Share, Cash Paid	$ 25.75	$ 26.07		$ 1.53	$ 1.51	$ 1.50
Proceeds from issuance of common stock	$ 198,228,000	$ 229,451,000		$ 540,560,000	$ 17,692,000	$ 68,060,000
Stock Issuance Costs	9,060,000	10,393,000		(19,453,000)	(1,000)	(113,000)
Common Stock, Shares Reserved for Future Issuance, Dividend Reinvestment Plan			16,000,000
DRIP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Proceeds from issuance of common stock				$ 93,451,000	$ 17,623,000	$ 67,354,000
Stock Issued During Period, Shares, Dividend Reinvestment Plan				3,745,896	793,759	3,766,452