Stock Plans and Stock Based Compensation - Options and SARs Outstanding and Exercisable (Details)	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Outstanding (in shares) | shares	269,506
Weighted average remaining contractual life (in years)	5 years 3 months 26 days
Weighted average exercise price (in dollars per share)	$ 47.34
Number exercisable (in shares) | shares	130,610
Weighted average exercise price (in dollars per share)	$ 41.88
$20.01-29.30
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise prices, lower range (in dollars per share)	20.01
Range of exercise prices, upper range (in dollars per share)	$ 29.30
Outstanding (in shares) | shares	1,734
Weighted average remaining contractual life (in years)	3 years 5 months 5 days
Weighted average exercise price (in dollars per share)	$ 21.65
Number exercisable (in shares) | shares	1,734
Weighted average exercise price (in dollars per share)	$ 21.65
$29.31-35.70
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise prices, lower range (in dollars per share)	29.31
Range of exercise prices, upper range (in dollars per share)	$ 35.70
Outstanding (in shares) | shares	626
Weighted average remaining contractual life (in years)	11 months 16 days
Weighted average exercise price (in dollars per share)	$ 30.96
Number exercisable (in shares) | shares	626
Weighted average exercise price (in dollars per share)	$ 30.96
$35.71-37.50
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise prices, lower range (in dollars per share)	35.71
Range of exercise prices, upper range (in dollars per share)	$ 37.50
Outstanding (in shares) | shares	48,855
Weighted average remaining contractual life (in years)	3 years 7 months 7 days
Weighted average exercise price (in dollars per share)	$ 37.00
Number exercisable (in shares) | shares	35,826
Weighted average exercise price (in dollars per share)	$ 37.00
$37.51-41.00
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise prices, lower range (in dollars per share)	37.51
Range of exercise prices, upper range (in dollars per share)	$ 41.00
Outstanding (in shares) | shares	38,447
Weighted average remaining contractual life (in years)	5 years 3 months 8 days
Weighted average exercise price (in dollars per share)	$ 40.60
Number exercisable (in shares) | shares	15,248
Weighted average exercise price (in dollars per share)	$ 40.60
$41.01-50.00
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise prices, lower range (in dollars per share)	41.01
Range of exercise prices, upper range (in dollars per share)	$ 50.00
Outstanding (in shares) | shares	115,236
Weighted average remaining contractual life (in years)	4 years 6 months 26 days
Weighted average exercise price (in dollars per share)	$ 45.92
Number exercisable (in shares) | shares	70,058
Weighted average exercise price (in dollars per share)	$ 43.79
$50.01-60.00
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise prices, lower range (in dollars per share)	50.01
Range of exercise prices, upper range (in dollars per share)	$ 60.00
Outstanding (in shares) | shares	45,117
Weighted average remaining contractual life (in years)	7 years 9 months 15 days
Weighted average exercise price (in dollars per share)	$ 56.29
Number exercisable (in shares) | shares	7,118
Weighted average exercise price (in dollars per share)	$ 56.29
$60.01-86.18
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise prices, lower range (in dollars per share)	60.01
Range of exercise prices, upper range (in dollars per share)	$ 86.18
Outstanding (in shares) | shares	19,491
Weighted average remaining contractual life (in years)	8 years 9 months 15 days
Weighted average exercise price (in dollars per share)	$ 77.01
Number exercisable (in shares) | shares	0
Weighted average exercise price (in dollars per share)