Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended		12 Months Ended
		Feb. 28, 2015	Nov. 30, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued during period, shares			5,462,500	31,807	34,230	5,493,595
Underwriters over allotment of shares (in shares)			712,500
Shares issued, price per share (in USD per share)			$ 38.16
Proceeds from issuance of common stock			$ 199,961	$ 278,040	$ 332,117	$ 360,072
Stock issuance costs			$ 8,488	$ 4,266	$ 4,178	10,683
Total allowable shares (in shares)				375,000,000	375,000,000
Total shares issued as of December 31, 2016 (in shares)				147,149,945	147,149,945
Net proceeds				$ 1,148	$ 991	209,240
Stock issuance costs				15,204	4,198	10,726
DRIP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Proceeds from issuance of common stock				$ 8,340	$ 7,182	$ 14,817
Shares reserved for future issuance, dividend reinvestment plan (in shares)		16,000,000
Shares of common stock (in shares)				187,626	196,584	422,406
2012 ATM Equity Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued during period, shares						0
2013 ATM Equity Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total allowable shares (in shares)				9,000,000
Total shares issued as of December 31, 2016 (in shares)				6,252,812
2015 ATM Equity Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total allowable shares (in shares)				10,000,000
Total shares issued as of December 31, 2016 (in shares)				9,852,465
2016 ATM Equity Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued during period, shares				5,716,222	8,573,533	3,758,362
Total allowable shares (in shares)				12,000,000
Total shares issued as of December 31, 2016 (in shares)				4,223,290
Average stock price per share (net) (in USD per share)				$ 46.48	$ 37.45	$ 35.90
Net proceeds				$ 265,696	$ 321,067	$ 134,919
Stock issuance costs	[1]			$ 4,266	$ 4,016	$ 2,195

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