Stockholders' Equity - At the Market Equity Offering Program (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Jul. 05, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Feb. 15, 2017
Class of Stock [Line Items]
Issuance of common stock (shares)	9,200,000
Net proceeds from issuance of common stock	$ 304,600	$ 450,503	$ 66,942	$ 76,856
At The Market Equity Offering Program [Member]
Class of Stock [Line Items]
Issuance of common stock (shares)		4,795,601	2,434,239	3,009,761
Net proceeds from issuance of common stock		$ 144,600	$ 65,800	$ 75,700
Minimum [Member] | At The Market Equity Offering Program [Member]
Class of Stock [Line Items]
Shares issued during period (USD per share)		$ 28.31	$ 25	$ 24.35
Maximum [Member] | At The Market Equity Offering Program [Member]
Class of Stock [Line Items]
Shares issued during period (USD per share)		$ 33.66	$ 29.15	$ 27.53
Subsequent Event [Member] | At The Market Equity Offering Program [Member]
Class of Stock [Line Items]
Number of authorized shares remaining under offering program (shares)					5,868,697