Stock Plans Stock Plans (Schedule of RSU Expense by Vesting Type) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 2,755	$ 1,097	$ 6,547	$ 3,759
Restricted stock unit awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	1,775	824	3,876	2,305
Restricted stock unit awards | Service-based
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	1,124	544	2,460	1,613
Restricted stock unit awards | Performance objectives
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	219	30	389	71
Restricted stock unit awards | Market-condition
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 432	$ 250	$ 1,027	$ 621