Share-Based Compensation - Weighted average assumptions used in valuing options - (Details)	12 Months Ended			90 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Jun. 30, 2009
Weighted-average assumptions
Dividend yield (as a percent)			1.40%
Expected life			8 years 6 months
Expected volatility (as a percent)			40.90%
Risk-free interest rate (as a percent)			1.79%
Employee Stock Purchase Plan
Additional disclosures
Compensation expense | $	$ 43,000	$ 48,000	$ 48,000
Employee Stock Purchase Plan
Number of shares of common stock registered in connection with the establishment of an Employee Stock Purchase Plan	363,825
Requisite age of employees to be eligible to participate in the plan	21 years
Requisite service period to be eligible to participate in the plan	6 months
Discount rate for eligible employees to purchase company stock (as a percent)	5.00%			15.00%
Minimum | Employee Stock Purchase Plan
Employee Stock Purchase Plan
Shares available for issuance under the plan	89,000
Maximum | Employee Stock Purchase Plan
Employee Stock Purchase Plan
Estimated subscription date fair value (as a percent)	95.00%
Stock Options
Weighted-average assumptions
Dividend yield (as a percent)	1.40%	1.60%
Expected life	8 years 6 months	8 years 6 months
Risk-free interest rate (as a percent)	2.43%	1.90%
Additional disclosures
Total unrecognized compensation cost related to non vested stock option grants | $	$ 1,400,000
Weighted-average period over which unrecognized compensation cost is expected to be recognized	1 year 1 month 17 days
Total fair value of shares vested during the period | $	$ 578,000	$ 455,000	386,000
Compensation expense | $	$ 784,000	$ 545,000	483,000
Stock Options | Minimum
Weighted-average assumptions
Expected volatility (as a percent)	37.20%	40.60%
Restricted Stock.
Additional disclosures
Weighted-average period over which unrecognized compensation cost is expected to be recognized	2 years 29 days
Compensation expense | $	$ 2,500,000	$ 2,700,000	$ 2,500,000
Restricted Stock Activity and RSUs
Nonvested at the beginning of the period (in shares)	183,014
Granted (in shares)	26,053	44,301	45,605
Vested (in shares)	(61,950)
Forfeited (in shares)	(4,425)
Nonvested at the end of the period (in shares)	142,692	183,014
Weighted-Average Grant-Date Fair Value
Nonvested at the beginning of the period (in dollars per share) | $ / shares	$ 51.88
Granted (in dollars per share) | $ / shares	89.11	$ 67.51	$ 70.86
Vested (in dollars per share) | $ / shares	47.94
Forfeited (in dollars per share) | $ / shares	75.59
Nonvested at the end of the period (in dollars per share) | $ / shares	$ 59.66	$ 51.88
Additional disclosures
Total unrecognized compensation cost related to nonvested restricted stock and RSUs granted | $	$ 4,500,000
Total fair value of shares vested during the period | $	$ 3,100,000	$ 3,200,000	$ 2,100,000
Restricted Stock. | Employees | Minimum
Additional disclosures
Vesting period	4 years
Restricted Stock. | Non-employee directors | Maximum
Additional disclosures
Vesting period	12 years
Restricted Stock Units
Additional disclosures
Weighted-average period over which unrecognized compensation cost is expected to be recognized	2 years 1 month 2 days
Compensation expense | $	$ 3,600,000	$ 2,900,000	$ 2,800,000
Restricted Stock Activity and RSUs
Nonvested at the beginning of the period (in shares)	107,876
Granted (in shares)	77,301	68,842	39,672
Vested (in shares)	(39,672)
Forfeited (in shares)	(1,518)
LTIP Adjustment (in shares)	3,951
Nonvested at the end of the period (in shares)	140,036	107,876
Weighted-Average Grant-Date Fair Value
Nonvested at the beginning of the period (in dollars per share) | $ / shares	$ 66.37
Granted (in dollars per share) | $ / shares	89.43
Vested (in dollars per share) | $ / shares	68.31
Forfeited (in dollars per share) | $ / shares	$ 78.68
LTIP adjustment (in dollars per share) | $ / shares	63.93
Nonvested at the end of the period (in dollars per share) | $ / shares	$ 78.49	$ 66.37
Additional disclosures
Total unrecognized compensation cost related to nonvested restricted stock and RSUs granted | $	$ 5,700,000
Total fair value of shares vested during the period | $	$ 5,000,000	$ 4,300,000	$ 1,900,000
Target RSU award level (as a percent)	97.10%