Stock-Based Compensation - Employee Stock Purchase Plan (Details) - USD ($) $ in Millions	1 Months Ended		12 Months Ended
	Jul. 31, 2018	Jan. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares issued for ESPP (in shares)	133,367	99,225	232,592	155,673
ESPP expense			$ 0.5	$ 0.4	$ 0.3
Employee Stock Purchase Plan | Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair market value at grant purchase price			85.00%
Fair market value at grant exercise price			85.00%
Common stock, reserved for future issuance (in shares)			3,000,000
Common stock, shares available for future issuance (in shares)			2,500,000