Statements of Changes in Stockholders’ Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statements of Changes in Stockholders’ Equity
Issuance of common stock in connection with employee stock option exercises, shares	65,611	1,311,239	24,498
Issuance of common stock in connection with restricted stock unit awards, shares	1,258,400		1,133,183
Issuance of common stock for the employee stock purchase plan, shares	61,239	74,551	95,981
Shares withheld for net settlements of share-based awards, shares	504,037	531,346	391,195
Purchase of common stock for treasury, shares	286,696	885,285	1,336,132
Net excess tax benefit			$ 20