Employee Stock Benefit Plans (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) plan $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Nov. 09, 2016 shares	Nov. 08, 2016 shares
Stock-based compensation
Number of stock-based employee compensation plans | plan	4
Number of executive stock award plans | plan	3
Total fair value of share awards vested | $	$ 2,370,000	$ 4,530,000	$ 4,370,000
Shares Available for Grant
Balance at the beginning of the period (in shares)	714,005	2,450,799	2,460,208
Granted (in shares)	(98,625)	(79,118)	(81,591)
Forfeited (in shares)	2,000	3,543	1,980
Canceled (in shares)		(1,950,000)
Balance at the end of the period (in shares)	676,444	714,005	2,450,799
2011 Stock Option Plan
Shares Available for Grant
Shares authorized	250,000				2,200,000
Stock award plans
Additional disclosures
Stock-based compensation expense | $	$ 2,960,000	$ 2,880,000	$ 3,840,000
Income tax benefit due to stock-based compensation expenses | $	1,110,000	$ 1,070,000	$ 1,450,000
Unrecognized stock-based compensation expense | $	$ 5,970,000
Weighted-average period for unrecognized compensation cost to recognize	3 years 1 month 11 days
Executive Incentive Plan
Shares Available for Grant
Shares authorized	59,064	59,342	70,202
Additional disclosures
Vesting period	5 years
1982 Restricted Stock Plan
Shares Available for Grant
Shares authorized		229,439		250,000
1982 Restricted Stock Plan | Minimum
Additional disclosures
Vesting period	2 years
1982 Restricted Stock Plan | Maximum
Additional disclosures
Vesting period	10 years
Strategic Deployment Incentive Plan
Additional disclosures
Vesting period	5 years
Stock options
Additional disclosures
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	0	0	0
Stock-based compensation expense | $	$ 0	$ 0	$ 0
Stock options | Maximum
Additional disclosures
Contractual life	10 years
Non-vested stock awards outstanding
Number of Shares
Stock awards outstanding at the beginning of the period (in shares)	276,615	358,861	440,035
Granted (in shares)	98,625	79,118	81,591
Vested (in shares)	(76,858)	(155,981)	(159,381)
Forfeited (in shares)	(2,456)	(5,383)	(3,384)
Stock awards outstanding at the end of the period (in shares)	295,926	276,615	358,861
Weighted-Average Grant-Date Fair Value
Stock awards outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 23.94	$ 21.93	$ 20.60
Granted (in dollars per share) | $ / shares	33.54	26.19	24.44
Vested (in dollars per share) | $ / shares	22.71	20.47	19.51
Forfeited (in dollars per share) | $ / shares	29.93	23.39	23.85
Stock awards outstanding at the end of the period (in dollars per share) | $ / shares	$ 27.41	23.94	21.93
Employee Stock Purchase Plan
Additional disclosures
Required period of service	2 years
Maximum base rate of employee basic pay to purchase any dollar amount of stock (as a percent)	25.00%
Maximum stock which can be purchased by employee per year | $	$ 25,000
Purchases price (in dollars per share) | $ / shares	$ 46.18	$ 33.87	$ 28.80
Percentage of the premium fair market value of common stock as on the offering date or the market value as on the purchase date (as a percent)			0.23%
Percentage of the discount fair market value of common stock as on the offering date or the market value as on the purchase date (as a percent)	(1.32%)	(0.29%)
ESPP offering through May 2019
Additional disclosures
Value of shares to be purchased | $	$ 152,786
Stock value of most recent offering (in dollars per share ) | $ / shares	$ 46.18