Common Stock (Details) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended	12 Months Ended						1 Months Ended
	Nov. 30, 2014	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012		May 31, 2012	Mar. 31, 2013	Feb. 29, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued during period, shares	5,462,500	5,493,595		29,013		40,460
Underwriters over allotment of shares	712,500	1,500,000
Shares Issued, price per share	$ 38.16
Proceeds from issuance of common stock	$ 199,961	$ 360,072		$ 267,613		$ 185,223
Stock issuance costs	8,488	10,683		3,774		2,265
Issuance of common stock		209,240		744		833
Stock issuance costs		10,726		13,529		12,237
2012 ATM Equity Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued during period, shares		0		4,676,542		4,282,298
Shares issued		8,958,840
Number of shares authorized to be issued								9,000,000
Average stock price per share				$ 32.60		$ 29.64
Issuance of common stock				152,435		126,947
Stock issuance costs				2,161	[1]	2,145	[1]
2013 ATM Equity Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued during period, shares		3,758,362		2,280,450
Shares issued		6,038,812
Number of shares authorized to be issued									9,000,000
Average stock price per share		$ 35.90		$ 37.80
Issuance of common stock		134,919		86,208
Stock issuance costs		2,195	[1]	1,613	[1]
DRIP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Proceeds from issuance of common stock		$ 14,817		$ 25,407		$ 56,102
Shares of common stock		422,406		764,891		2,101,644
Shares reserved for future issuance, dividend reinvestment plan										16,000,000

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