Employee Stock Benefit Plans (Details) (USD $)	12 Months Ended
	Dec. 31, 2013 plan	Dec. 31, 2012	Dec. 31, 2011
Stock-based compensation
Number of stock-based employee compensation plan	4
Number of executive stock award plans	3
Total fair value of share awards vested	$ 1,970,000	$ 4,300,000	$ 2,450,000
Shares Available for Grant
Balance at the beginning of the period (in shares)	2,305,116	2,320,492	2,319,648
Granted (in shares)	(88,980)	(98,617)	(261,523)
Forfeited (in shares)	5,642	4,124	1,029
Canceled (in shares)		0	(2,129,177)
Balance at the end of the period (in shares)	2,285,758	2,305,116	2,320,492
2011 Stock Option Plan
Shares Available for Grant
Shares authorized	2,000,000	2,000,000	2,000,000
Stock award plans
Additional disclosures
Stock-based compensation expense	2,900,000	2,070,000	2,090,000
Income tax benefit due to stock-based compensation expenses	1,100,000	780,000	790,000
Unrecognized stock-based compensation expense	6,090,000
Weighted-average period for unrecognized compensation cost to recognize	3 years 0 months 30 days
Executive Incentive Plan
Shares Available for Grant
Shares authorized	61,970	76,815	190,515
Additional disclosures
Vesting period	5 years
1982 Restricted Stock Plan
Shares Available for Grant
Shares authorized			100,000
1982 Restricted Stock Plan | Minimum
Additional disclosures
Vesting period	2 years
1982 Restricted Stock Plan | Maximum
Additional disclosures
Vesting period	10 years
1998 Performance Compensation Plan
Shares Available for Grant
Shares authorized	2,010	2,302	100,000
Additional disclosures
Vesting period	5 years
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)	$ 24.32	$ 20.54	$ 20.29
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)	1.82%	0.15%	0.00%
ESPP offering through May 2015
Additional disclosures
Value of shares to be purchased	$ 180,190
Stock value of most recent offering (in dollars per share )	$ 24.32
Non-vested stock awards outstanding
Number of Shares
Stock awards outstanding at the beginning of the period (in shares)	433,204	530,848	438,610
Granted (in shares)	88,980	98,617	261,523
Vested (in shares)	(85,985)	(190,674)	(121,744)
Forfeited (in shares)	(10,754)	(5,587)	(47,541)
Stock awards outstanding at the end of the period (in shares)	425,445	433,204	530,848
Weighted-Average Grant-Date Fair Value
Stock awards outstanding at the beginning of the period (in dollars per share)	$ 20.15	$ 18.76	$ 16.92
Granted (in dollars per share)	$ 24.19	$ 21.95	$ 20.15
Vested (in dollars per share)	$ 19.58	$ 17.24	$ 17.08
Forfeited (in dollars per share)	$ 20.71	$ 19.71	$ 13.71
Stock awards outstanding at the end of the period (in dollars per share)	$ 21.09	$ 20.15	$ 18.76
Stock options
Number of Shares
Options outstanding at the beginning of the period (in shares)	7,500	22,000	62,508
Exercised (in shares)	(7,500)	(14,500)	(5,090)
Forfeited (in shares)		0	(35,418)
Options outstanding at the end of the period (in shares)	0	7,500	22,000
Weighted Average Exercise Price
Options outstanding at the beginning of the period (in dollars per share)	$ 12.04	$ 12.04	$ 17.18
Exercised (in dollars per share)	$ 12.04	$ 12.04	$ 19.15
Forfeited (in dollars per share)		$ 0.00	$ 20.09
Options outstanding at the end of the period (in dollars per share)	$ 0.00	$ 12.04	$ 12.04
Stock options | Maximum
Additional disclosures
Contractual life	10 years