Share-Based Compensation - Weighted average assumptions used in valuing options - (Details)	6 Months Ended
	Jun. 30, 2018 USD ($) $ / shares shares	Jun. 30, 2017
Stock Options
Weighted-average assumptions
Dividend yield (as a percent)	1.46%	1.40%
Expected life	8 years 6 months	8 years 6 months
Expected volatility (as a percent)	28.00%	37.20%
Risk-free interest rate (as a percent)	2.54%	2.43%
Additional disclosures
Total unrecognized compensation cost related to non vested stock option grants | $	$ 1,800,000
Weighted-average period over which unrecognized compensation cost is expected to be recognized	1 year 7 months 13 days
Total fair value of shares vested during the period | $	$ 700,000
Restricted Stock.
Additional disclosures
Weighted-average period over which unrecognized compensation cost is expected to be recognized	1 year 9 months 15 days
Restricted Stock Activity and RSUs
Nonvested at the beginning of the period (in shares)	142,692
Granted (in shares)	7,836
Vested (in shares)	(18,688)
Nonvested at the end of the period (in shares)	131,840
Weighted-Average Grant-Date Fair Value
Nonvested at the beginning of the period (in dollars per share) | $ / shares	$ 59.66
Granted (in dollars per share) | $ / shares	87.37
Vested (in dollars per share) | $ / shares	66.61
Nonvested at the end of the period (in dollars per share) | $ / shares	$ 60.32
Additional disclosures
Total unrecognized compensation cost related to nonvested restricted stock and RSUs granted | $	$ 3,800,000
Total fair value of shares vested during the period | $	$ 1,400,000
Restricted Stock. | Employees | Minimum
Additional disclosures
Vesting period	4 years
Restricted Stock. | Non-employee directors | 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	2 years 26 days
Restricted Stock Activity and RSUs
Nonvested at the beginning of the period (in shares)	140,036
Granted (in shares)	100,186
Vested (in shares)	(1,176)
Forfeited (in shares)	(217)
LTIP Adjustment (in shares)	3,213
Nonvested at the end of the period (in shares)	235,616
Weighted-Average Grant-Date Fair Value
Nonvested at the beginning of the period (in dollars per share) | $ / shares	$ 78.49
Granted (in dollars per share) | $ / shares	86.34
Vested (in dollars per share) | $ / shares	84.35
Forfeited (in dollars per share) | $ / shares	$ 92.05
LTIP adjustment (in dollars per share) | $ / shares	89.40
Nonvested at the end of the period (in dollars per share) | $ / shares	$ 81.64
Additional disclosures
Total unrecognized compensation cost related to nonvested restricted stock and RSUs granted | $	$ 10,600,000
Total fair value of shares vested during the period | $	$ 2,700,000
Target RSU award level (as a percent)	83.00%
Employee Stock Purchase Plan
Number Of Stock Issued Pursuant To Restricted Stock Units	38,365
Number of nonvested restricted stock issued	15,836
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
Discretionary cliff vesting restricted stock units
Additional disclosures
Performance period	4 years