Stock Plans Stock Plans (Schedule of RSU Expense by Vesting Type) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 2,624	$ 1,538
Restricted stock unit awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	1,502	865
Restricted stock unit awards | Service-based
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	1,084	667
Restricted stock unit awards | Performance objectives
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	120	6
Restricted stock unit awards | Market-condition
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 298	$ 192