Stock-Based Compensation - Equity Incentive Plans (Details) - shares		3 Months Ended
	Jul. 26, 2013	Mar. 31, 2018	Dec. 31, 2017
Class of Stock
Common stock options outstanding		24,217,548	21,402,200
Options outstanding under earlier plans		4,138,937
Grant of inducement stock option		3,321,875
2013 Stock Incentive Plan
Class of Stock
Maximum number of additional common shares.	6,946,978
Common stock options outstanding		16,756,736
Common shares available for grant	25,224,460	10,114,740