Continuous Equity Offering Continuous Equity Offering (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Class of Stock [Line Items]
Net proceeds from issuance of common shares	$ 44,927,000	$ 16,197,000
Continuous Equity Offering [Member]
Class of Stock [Line Items]
Common stock, Continuous equity offering program, Max amount of equity	200,000,000
Net proceeds from issuance of common shares	$ 24,300,000
Continuous Equity Offering [Member] | Common Stock [Member]
Class of Stock [Line Items]
Stock Issued During Period, Shares, Continuous Equity Offering	642,000