Stock Plans (Summary of Stock Option Activity) (Details) shares in Thousands	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Stock option activity
Shares outstanding at beginning of the year	2,287
Shares granted	464
Shares exercised	(578)
Shares forfeited	(19)
Shares outstanding at end of year	2,154
Shares exercisable at end of year	1,229
Number of shares vested and expected to vest in the future	2,200
Weighted-average price, outstanding at beginning of year (per share) | $ / shares	$ 37.64
Weighted-average price, granted (per share) | $ / shares	53.94
Weighted-average price, exercised (per share) | $ / shares	31.16
Weighted-average price, forfeited (per share) | $ / shares	50.86
Weighted-average price, outstanding at end of year (per share) | $ / shares	42.77
Weighted-average price, exercisable at end of year (per share) | $ / shares	$ 37.79