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		1 Months Ended	12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	May 31, 2012 ATM Equity Program	Dec. 31, 2013 ATM Equity Program		Dec. 31, 2012 ATM Equity Program		Mar. 31, 2013 2013 ATM Equity Program	Dec. 31, 2013 2013 ATM Equity Program		Feb. 29, 2012 DRIP	Dec. 31, 2013 DRIP	Dec. 31, 2012 DRIP	Dec. 31, 2011 DRIP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued during period, shares	8,050,000	9,200,000	29,013	40,460	17,288,265		4,676,542		4,282,298			2,280,450
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	$ 267,613	$ 185,223	$ 540,560										$ 25,407	$ 56,102	$ 93,451
Stock issuance costs	9,060	10,393	3,774	2,265	19,453
Number of shares authorized to be issued						9,000,000					9,000,000
Average stock price per share							$ 32.60		$ 29.64			$ 37.80
Issuance of common stock			744	833	447,863		152,435		126,947			86,208
Stock issuance costs			$ 13,529	$ 12,237	$ 19,328		$ 2,161	[1]	$ 2,145	[1]		$ 1,613	[1]
Shares reserved for future issuance, dividend reinvestment plan														16,000,000
Shares of common stock															764,891	2,101,644	3,745,896

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