Statements of Changes in Stockholders’ Equity (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statements of Changes in Stockholders’ Equity
Issuance of common stock in connection with employee stock option exercises, shares		24,498	39,002
Issuance of common stock in connection with restricted stock unit awards, shares	1,311,239	1,133,183	1,159,047
Net excess tax benefit		$ 0.2	$ 2.1
Issuance of common stock for the employee stock purchase plan, shares	74,551	95,981	70,924
Shares withheld for net settlements of share-based awards, shares	531,346	391,195	418,135
Purchase of common stock for treasury, shares	885,285	1,336,132	1,992,831