Stockholders' Equity - At the Market Equity Offering Program (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	12 Months Ended
	Jul. 19, 2013	Jan. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 23, 2015	Mar. 29, 2013
Class of Stock [Line Items]
Issuance of common stock (shares)	3,000,000
Net proceeds from issuance of common stock	$ 78,300		$ 66,942	$ 76,856	$ 220,252
At The Market Equity Offering Program [Member]
Class of Stock [Line Items]
Shares authorized to be sold (shares)						2,500,000	9,000,000
Issuance of common stock (shares)			2,434,239	3,009,761	5,207,871
Net proceeds from issuance of common stock			$ 65,800	$ 75,700	$ 140,600
Number of authorized shares remaining under offering program			2,447,400
Minimum [Member] | At The Market Equity Offering Program [Member]
Class of Stock [Line Items]
Issuance of common stock (price per share)			$ 25.00	$ 24.35	$ 24.19
Maximum [Member] | At The Market Equity Offering Program [Member]
Class of Stock [Line Items]
Issuance of common stock (price per share)			$ 29.15	$ 27.53	$ 30.49
Subsequent Event [Member] | At The Market Equity Offering Program [Member]
Class of Stock [Line Items]
Issuance of common stock (shares)		664,298
Net proceeds from issuance of common stock		$ 18,700