Continuous Equity Offering Continuous Equity Offering (Details) (USD $) Share data in Millions, unless otherwise specified	6 Months Ended		3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 Continuous Equity Offering [Member]	Jun. 30, 2013 Continuous Equity Offering [Member]	Jun. 30, 2013 Continuous Equity Offering [Member] Common Stock [Member]	Jun. 30, 2013 Continuous Equity Offering [Member] Common Stock [Member]
Class of Stock [Line Items]
Common stock, Continuous equity offering program, Max amount of equity			$ 200,000,000	$ 200,000,000
Stock Issued During Period, Shares, Continuous Equity Offering					1.2	1.9
Net proceeds from issuance of common shares	$ 108,852,000	$ 31,234,000	$ 50,400,000	$ 75,000,000