Share-Based Compensation - Weighted average assumptions used in valuing options (Details)	3 Months Ended
	Mar. 31, 2020 USD ($) $ / shares shares
Stock Options
Share-Based Compensation
Shares issued (in shares)	0
Additional disclosures
Total unrecognized compensation cost related to non vested stock option grants | $	$ 600,000
Weighted-average period over which unrecognized compensation cost is expected to be recognized	1 year 1 month 24 days
Total fair value of shares vested during the period | $	$ 641,000
Restricted Stock
Additional disclosures
Weighted-average period over which unrecognized compensation cost is expected to be recognized	2 years 6 months 7 days
Total fair value of shares vested during the period | $	$ 2,800,000
Restricted Stock Activity and RSUs
Nonvested at the beginning of the period (in shares)	69,450
Granted (in shares)	815
Vested (in shares)	(37,359)
Forfeited (in shares)	(317)
Nonvested at the end of the period (in shares)	32,589
Weighted-Average Grant-Date Fair Value
Nonvested at the beginning of the period (in dollars per share) | $ / shares	$ 58.96
Granted (in dollars per share) | $ / shares	86.75
Vested (in dollars per share) | $ / shares	72.65
Forfeited (in dollars per share) | $ / shares	65.86
Nonvested at the end of the period (in dollars per share) | $ / shares	$ 43.89
Additional disclosures
Total unrecognized compensation cost related to nonvested restricted stock and RSUs granted | $	$ 850,000
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 6 months 21 days
Total fair value of shares vested during the period | $	$ 2,800,000
Restricted Stock Activity and RSUs
Nonvested at the beginning of the period (in shares)	324,601
Granted (in shares)	19,424
Forfeited (in shares)	(2,830)
Nonvested at the end of the period (in shares)	341,195
Weighted-Average Grant-Date Fair Value
Nonvested at the beginning of the period (in dollars per share) | $ / shares	$ 76.44
Granted (in dollars per share) | $ / shares	86.14
Forfeited (in dollars per share) | $ / shares	81.39
Nonvested at the end of the period (in dollars per share) | $ / shares	$ 76.95
Additional disclosures
Total unrecognized compensation cost related to nonvested restricted stock and RSUs granted | $	$ 10,400,000
Target RSU award level (as a percent)	85.20%
Employee Stock Purchase Plan
Number Of Stock Issued Pursuant To Restricted Stock Units	31,435
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