Share-Based Compensation - Weighted average assumptions used in valuing options (Details)	9 Months Ended
	Sep. 30, 2019 USD ($) $ / shares shares	Sep. 30, 2018
Stock Options
Share-Based Compensation
Shares issued (in shares)	0
Weighted-average assumptions
Dividend yield (as a percent)		1.46%
Expected life		8 years 6 months
Expected volatility (as a percent)		28.00%
Risk-free interest rate (as a percent)		2.54%
Additional disclosures
Total unrecognized compensation cost related to non vested stock option grants | $	$ 880,000
Weighted-average period over which unrecognized compensation cost is expected to be recognized	1 year 3 days
Total fair value of shares vested during the period | $	$ 799,000
Restricted Stock
Additional disclosures
Weighted-average period over which unrecognized compensation cost is expected to be recognized	1 year 4 months 13 days
Total fair value of shares vested during the period | $	$ 2,600,000
Restricted Stock Activity and RSUs
Nonvested at the beginning of the period (in shares)	104,419
Granted (in shares)	8,934
Vested (in shares)	(33,509)
Forfeited (in shares)	(2,332)
Nonvested at the end of the period (in shares)	77,512
Weighted-Average Grant-Date Fair Value
Nonvested at the beginning of the period (in dollars per share) | $ / shares	$ 62.45
Granted (in dollars per share) | $ / shares	73.34
Vested (in dollars per share) | $ / shares	73.78
Forfeited (in dollars per share) | $ / shares	73.89
Nonvested at the end of the period (in dollars per share) | $ / shares	$ 58.47
Additional disclosures
Total unrecognized compensation cost related to nonvested restricted stock and RSUs granted | $	$ 1,500,000
Restricted Stock | Maximum
Additional disclosures
Vesting period	12 months
Restricted Stock Units
Additional disclosures
Weighted-average period over which unrecognized compensation cost is expected to be recognized	1 year 9 months 29 days
Total fair value of shares vested during the period | $	$ 2,900,000
Restricted Stock Activity and RSUs
Nonvested at the beginning of the period (in shares)	200,540
Granted (in shares)	153,060
Forfeited (in shares)	(2,154)
Nonvested at the end of the period (in shares)	351,446
Weighted-Average Grant-Date Fair Value
Nonvested at the beginning of the period (in dollars per share) | $ / shares	$ 85.10
Granted (in dollars per share) | $ / shares	67.86
Forfeited (in dollars per share) | $ / shares	87.36
Nonvested at the end of the period (in dollars per share) | $ / shares	$ 77.58
Additional disclosures
Total unrecognized compensation cost related to nonvested restricted stock and RSUs granted | $	$ 11,800,000
Target RSU award level (as a percent)	83.30%
Employee Stock Purchase Plan
Number Of Stock Issued Pursuant To Restricted Stock Units	44,599
Performance based restricted stock units
Additional disclosures
Performance period	3 years
Other Performance based restricted stock units
Additional disclosures
Performance period	1 year
Timed based restricted stock units
Additional disclosures
Performance period	4 years