Stockholders’ Equity - Common Stock (Details) $ / shares in Units, $ in Millions							12 Months Ended
	Nov. 01, 2017 USD ($) shares	Oct. 30, 2017 USD ($) person $ / shares shares	Oct. 28, 2016 USD ($) person $ / shares shares	Oct. 13, 2016 USD ($) $ / shares shares	Feb. 19, 2015 USD ($) person $ / shares shares	May 05, 1998 $ / shares shares	Dec. 31, 2017 $ / shares shares	Dec. 31, 2016 shares
Class of Stock
Common stock, shares authorized							280,000,000	280,000,000
Common stock, shares reserved for future issuance							78,662,283
Common Stock
Class of Stock
Stock issued (in shares)	5,000,000	33,333,334	1,225,243	25,000,000	23,000,000	1,199,684
Sale price of common shares subject to the liquidation put | $ / shares						$ 16.00
Minimum closing sales price of common stock to issue termination notice to put holders | $ / shares							$ 32.00
Price exceeds on number of days							20 days
Liquidation put shares no longer held by original holders							399,950
Shares held by put holders							35,780
Put Rights outstanding							0
Remaining shares of common stock subject to the liquidation put							763,954
Number of days underwriters have to purchase additional shares	30 days		30 days
Net proceeds from offering of common stock | $	$ 53.7		$ 48.8		$ 80.6
Stock price (in dollars per share) | $ / shares		$ 1.50		$ 2.00	$ 3.75
Gross proceeds from offering of common stock | $		$ 50.0		$ 50.0	$ 86.3
Common Stock | Investor
Class of Stock
Number of people | person			2
Common Stock | Investor | Board of Director
Class of Stock
Stock issued (in shares)			5,125,000		5,333,333
Stock price (in dollars per share) | $ / shares			$ 2.00		$ 3.75
Number of people | person					2
Common Stock | Baker Bros. Advisors LP
Class of Stock
Stock issued (in shares)		8,000,000
Number of people | person		2