Equity Financings (Details) - USD ($)	1 Months Ended			12 Months Ended			20 Months Ended
	May 31, 2015	Apr. 30, 2015	May 31, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2015
Equity Financings
Proceeds from public offering of common stock, net of issuance costs				$ 0	$ 0	$ 148,439,000
Aggregate sales proceeds from issue and sell of shares of common stock under sales agreement with Stifel				$ 64,090,000	$ 0	$ 64,503,000
Commission percentage paid		2.00%
Underwritten public offering
Equity Financings
Aggregate common stock sold (in shares)	8,337,500
Share price (in dollars per share)	$ 19.00
Proceeds from public offering of common stock, net of issuance costs	$ 148,400,000
ATM 2014
Equity Financings
Aggregate sales proceeds from issue and sell of shares of common stock under sales agreement with Stifel							$ 73,480,764.000000
Commission percentage paid			2.00%
Aggregate stock sold pursuant to the sales agreement (in shares)						3,800,000	5,400,000
Aggregate receivable from sale of common stock under sales agreement with Stifel						$ 55,200,000
ATM 2015
Equity Financings
Aggregate stock sold pursuant to the sales agreement (in shares)				4,500,000		500,000
Aggregate receivable from sale of common stock under sales agreement with Stifel				$ 64,100,000		$ 9,300,000
Maximum
Equity Financings
Aggregate sales proceeds from issue and sell of shares of common stock under sales agreement with Stifel		$ 75,000,000
Maximum | ATM 2014
Equity Financings
Aggregate sales proceeds from issue and sell of shares of common stock under sales agreement with Stifel			$ 75,000,000