Stockholders' Equity	12 Months Ended
Dec. 31, 2017
Equity [Abstract]
Stockholders' Equity	Stockholders' Equity The Company's amended and restated certificate of incorporation authorizes 125,000,000 shares of common stock, and 5,000,000 shares of undesignated preferred stock, both with a par value of $0.01 per share. There were no shares of undesignated preferred stock issued or outstanding as of December 31, 2017 or 2016. In February 2014, the Company completed an equity offering, in which the Company sold 1,800,000 shares of its common stock at a price of $36.50 per share. Additionally, the underwriters of the offering exercised the full amount of their over-allotment option resulting in the sale of an additional 450,000 shares of the Company's common stock at a price of $36.50 per share. The Company received proceeds of $76.7 million from this offering, net of underwriting discounts and commissions and other offering expenses. In January 2015, the Company's stock purchase agreement and investor agreement, each with Johnson & Johnson Innovation – JJDC, Inc. (JJDC) became effective (see Note 9 for additional information). Under these agreements, JJDC purchased 1,923,077 new shares of the Company's common stock at a price of $39.00 per share, representing proceeds of $75.0 million. In July 2015, the Company completed an equity offering, in which the Company sold 3,525,000 shares of its common stock at a price of $37.00 per share. Additionally, the underwriters of the offering exercised the full amount of their over-allotment option resulting in the sale of an additional 528,750 shares of the Company's common stock at a price of $37.00 per share. The Company received net proceeds of $141.0 million from this offering, net of underwriting discounts and commissions and other estimated offering expenses. On April 26, 2017, the Company entered into a definitive agreement with an institutional healthcare investor to purchase 1,100,000 shares of its common stock at a purchase price of $21.50 per share in a registered direct offering. Proceeds to the Company, before deducting estimated offering expenses, were $23.7 million. The shares were offered pursuant to the Company’s effective shelf registration on Form S-3 that was filed with the SEC on November 2, 2016. On May 3, 2017, the Company entered into a sales agreement with an agent to sell, from time to time, shares of its common stock having an aggregate sales price of up to $75.0 million through an “at the market offering” (ATM Offering) as defined in Rule 415 under the Securities Act of 1933, as amended. The shares that may be sold under the sales agreement would be issued and sold pursuant to the Company's shelf registration statement on Form S-3 that was filed with the SEC on November 2, 2016. During the year ended December 31, 2017, the Company sold 599,284 shares of common stock resulting in net proceeds of $10.8 million related to the ATM Offering.

Stockholders' Equity

The Company's amended and restated certificate of incorporation authorizes 125,000,000 shares of common stock, and 5,000,000 shares of undesignated preferred stock, both with a par value of $0.01 per share. There were no shares of undesignated preferred stock issued or outstanding as of December 31, 2017 or 2016.

In February 2014, the Company completed an equity offering, in which the Company sold 1,800,000 shares of its common stock at a price of $36.50 per share. Additionally, the underwriters of the offering exercised the full amount of their over-allotment option resulting in the sale of an additional 450,000 shares of the Company's common stock at a price of $36.50 per share. The Company received proceeds of $76.7 million from this offering, net of underwriting discounts and commissions and other offering expenses.

In January 2015, the Company's stock purchase agreement and investor agreement, each with Johnson & Johnson Innovation – JJDC, Inc. (JJDC) became effective (see Note 9 for additional information). Under these agreements, JJDC purchased 1,923,077 new shares of the Company's common stock at a price of $39.00 per share, representing proceeds of $75.0 million.

In July 2015, the Company completed an equity offering, in which the Company sold 3,525,000 shares of its common stock at a price of $37.00 per share. Additionally, the underwriters of the offering exercised the full amount of their over-allotment option resulting in the sale of an additional 528,750 shares of the Company's common stock at a price of $37.00 per share. The Company received net proceeds of $141.0 million from this offering, net of underwriting discounts and commissions and other estimated offering expenses.

On April 26, 2017, the Company entered into a definitive agreement with an institutional healthcare investor to purchase 1,100,000 shares of its common stock at a purchase price of $21.50 per share in a registered direct offering. Proceeds to the Company, before deducting estimated offering expenses, were $23.7 million. The shares were offered pursuant to the Company’s effective shelf registration on Form S-3 that was filed with the SEC on November 2, 2016.

On May 3, 2017, the Company entered into a sales agreement with an agent to sell, from time to time, shares of its common stock having an aggregate sales price of up to $75.0 million through an “at the market offering” (ATM Offering) as defined in Rule 415 under the Securities Act of 1933, as amended. The shares that may be sold under the sales agreement would be issued and sold pursuant to the Company's shelf registration statement on Form S-3 that was filed with the SEC on November 2, 2016. During the year ended December 31, 2017, the Company sold 599,284 shares of common stock resulting in net proceeds of $10.8 million related to the ATM Offering.