Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended		11 Months Ended	12 Months Ended			34 Months Ended	44 Months Ended
		Feb. 28, 2015	Nov. 30, 2014	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Underwriters over allotment of shares (in shares)			712,500
Shares Issued, price per share			$ 38.16
Proceeds from issuance of common stock			$ 199,961		$ 332,117	$ 360,072	$ 267,613
Stock issuance costs			$ 8,488		$ 4,178	$ 10,683	$ 3,774
Stock issued during period, shares			5,462,500		34,230	5,493,595	29,013
Issuance of common stock					$ 991	$ 209,240	$ 744
Stock issuance costs					$ 4,198	$ 10,726	$ 13,529
2012 ATM Equity Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized to be issued					9,000,000
Stock issued during period, shares					0	0	4,676,542		8,958,840
Average stock price per share							$ 32.60
Issuance of common stock							$ 152,435
Stock issuance costs	[1]						$ 2,161
2013 ATM Equity Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized to be issued					9,000,000
Stock issued during period, shares					214,000	3,758,362	2,280,450	6,252,812
Average stock price per share					$ 39.84	$ 35.90	$ 37.80
Issuance of common stock					$ 8,525	$ 134,919	$ 86,208
Stock issuance costs	[1]				$ 130	2,195	1,613
2015 ATM Equity Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized to be issued					10,000,000
Stock issued during period, shares				8,359,533	8,359,533
Average stock price per share					$ 37.39
Issuance of common stock					$ 312,542
Stock issuance costs	[1]				3,886
DRIP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Proceeds from issuance of common stock					$ 7,182	$ 14,817	$ 25,407
Shares reserved for future issuance, dividend reinvestment plan		16,000,000
Shares of common stock					196,584	422,406	764,891

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