Common Stock (Details) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended		12 Months Ended			1 Months Ended	12 Months Ended		1 Months Ended	12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	May 31, 2012 ATM Equity Program [Member]	Dec. 31, 2012 ATM Equity Program [Member]		Feb. 29, 2012 DRIP [Member]	Dec. 31, 2012 DRIP [Member]	Dec. 31, 2011 DRIP [Member]	Dec. 31, 2010 DRIP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued during period, shares	8,050,000	9,200,000	40,460	17,288,265	39,872		4,282,298
Underwriters over allotment Of shares	1,050,000	1,200,000
Shares Issued, price per share	$ 25.75	$ 26.07		$ 25.75
Proceeds from issuance of common stock	$ 198,228	$ 229,451	$ 185,223	$ 540,560	$ 17,692					$ 56,102	$ 93,451	$ 17,623
Stock issuance costs	9,060	10,393	2,265	19,453	1
Number of shares authorized to be issued						9,000,000
Equity program, period in force						3 years
Average stock price per share							$ 29.64
Issuance of common stock			833	447,863	698		126,947
Stock issuance costs			$ 12,237	$ 19,328	$ 1		$ 2,145	[1]
Shares reserved for future issuance, dividend reinvestment plan									16,000,000
Shares of common stock										2,101,644	3,745,896	793,759

[1] Stock issuance costs consist primarily of underwriters' fees and commissions, and legal and accounting fees.