Summary of Significant Accounting Policies - 401(k) Defined Contribution Plan (Details) - 401(k) Defined Contribution Plan - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Contribution Plan Disclosure [Line Items]
Employer matching contribution (percent)	100.00%	100.00%	100.00%
Maximum annual contributions per employee (percent)	10.00%	6.00%	6.00%
Maximum annual contributions per employee	$ 4,000	$ 3,000	$ 3,000
Compensation expense	$ 2,700,000	$ 1,800,000	$ 1,200,000