Stockholders' Equity - At the Market Equity Offering Program (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Feb. 14, 2018 shares	Feb. 19, 2016 investment_bank shares	Feb. 17, 2016 investment_bank
Class of Stock [Line Items]
Proceeds from Issuance of Common Stock | $	$ 248,554	$ 450,503	$ 66,942
At The Market Equity Offering Program [Member]
Class of Stock [Line Items]
Equity offering program, number of investment banks in sales agreements | investment_bank					5
Equity offering program, shares authorized to be sold					10,000,000
Issuance of common stock (shares)	0	4,795,601	2,434,239
Proceeds from Issuance of Common Stock | $	$ 0	$ 144,600	$ 65,800
Previous At the Market Equity Offering Program [Member]
Class of Stock [Line Items]
Equity offering program, number of investment banks in sales agreements | investment_bank						1
Minimum [Member] | At The Market Equity Offering Program [Member]
Class of Stock [Line Items]
Shares issued during period (USD per share) | $ / shares	$ 0	$ 28.31	$ 25
Maximum [Member] | At The Market Equity Offering Program [Member]
Class of Stock [Line Items]
Shares issued during period (USD per share) | $ / shares	$ 0	$ 33.66	$ 29.15
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