401(K) Savings Plan (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
401(k) Savings Plan
Employer match (as a percent)	200.00%
Maximum employee contributions eligible for matching contributions	$ 6,000
Maximum employee contributions eligible for matching contributions (as a percent)	3.00%
Contributions to the 401(k) Plan	$ 500,000	$ 0	$ 0