Stock Plans - Restricted Stock Units - Vesting (Details) - Restricted Stock Units (RSUs)	12 Months Ended
	Dec. 31, 2018
Employee | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period from the date of grant for restricted stock awards granted	1 year
Employee | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period from the date of grant for restricted stock awards granted	4 years
Non-Employee Director
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period from the date of grant for restricted stock awards granted	1 year