Capital Transactions (Details 2)		12 Months Ended
	May. 04, 2010 shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) $ / shares shares	Dec. 31, 2013 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) plan $ / shares shares	Dec. 31, 2014 $ / shares shares
Stock Compensation
Additional shares of common stock available for issuance					0
Compensation expenses
Stock based compensation expense | $		$ 5,368,000	$ 5,318,000	$ 4,363,000
Maximum
Assumptions used to determine value of the shares purchased under the stock purchase plan using Black-Scholes option pricing model
Award percentage		150.00%
Minimum
Assumptions used to determine value of the shares purchased under the stock purchase plan using Black-Scholes option pricing model
Award percentage		0.00%
Stock Options and Restricted Stock
Compensation expenses
Total unrecognized compensation cost | $					$ 8,700,000
Unrecognized compensation cost recognized over a weighted average period (in years)		2 years
Employee Stock Option
Stock Compensation
Options outstanding (in shares)		2,888,074	3,407,575	3,653,859	1,064,429	2,888,074
Assumptions used to determine value of the shares purchased under the stock purchase plan using Black-Scholes option pricing model
Expected annual dividend rate (as a percent)		1.18%	1.32%	1.21%
Risk-free interest rate (as a percent)		1.62%	1.50%	0.92%
Average expected life (years)		5 years 8 months 12 days	5 years 9 months 18 days	6 years
Expected volatility of common stock (as a percent)		0.401%	0.431%	0.446%
Forfeiture rate (as a percent)		0.00%	0.00%	0.00%
Weighted average fair value of option grants | $ / shares		$ 4.75	$ 4.47	$ 5.04
Weighted Average Grant-Date Fair Value
Granted (in dollars per share) | $ / shares		$ 4.92
Compensation expenses
Period from grant date after which stock options become exercisable, minimum (in months)		6 months
Period from grant date within which incentive stock options are exercisable, maximum (in years)		7 years
Number of Options
Outstanding at the beginning of the period (in shares)		2,888,074	3,407,575	3,653,859
Granted (in shares)		267,433	255,503	173,560
Exercised (in shares)		(544,917)	(6,701)	(75,080)
Cancelled (in shares)		(1,546,161)	(768,303)	(344,764)
Outstanding at the end of the period (in shares)		1,064,429	2,888,074	3,407,575
Exercisable (in shares)					484,747	2,324,336
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share) | $ / shares		$ 16.46	$ 17.99	$ 18.44
Granted (in dollars per share) | $ / shares		13.63	11.96	13.24
Exercised (in dollars per share) | $ / shares		14.68	12.10	10.91
Cancelled (in dollars per share) | $ / shares		18.53	6.81	20.67
Outstanding at the end of the period (in dollars per share) | $ / shares		$ 13.64	$ 16.46	$ 17.99
Exercisable (in dollars per share) | $ / shares					$ 14.46	$ 17.39
Weighted Average Remaining Contractual Term
Outstanding, Weighted Average Remaining Contractual Term (in years)		3 years 8 months 12 days	1 year 8 months 12 days
Exercisable, Weighted Average Remaining Contractual Term (in years)		1 year 8 months 12 days	9 months 18 days
Aggregate Intrinsic Value
Average intrinsic value of shares outstanding | $					$ 5,726,700
Average intrinsic value of shares exercisable | $					$ 2,212,500
Total intrinsic value of options exercised | $		$ 1,800,000	$ 30,000	$ 172,000
Non-vested stock options
Non-vested shares at beginning of year (in shares)		563,738
Granted (in shares)		267,433	255,503	173,560
Vested (in shares)		(187,403)
Cancelled (in shares)		(64,086)
Non-vested shares at end of year (in shares)		579,682	563,738
Weighted Average Grant-Date Fair Value
Non-vested shares at beginning of year (in dollars per share) | $ / shares		$ 4.56
Weighted average fair value of option grants | $ / shares		4.75	$ 4.47	$ 5.04
Vested (in dollars per share) | $ / shares		4.43
Cancelled (in dollars per share) | $ / shares		4.65
Non-vested shares at end of year (in dollars per share) | $ / shares		$ 4.75	$ 4.56
Restricted Stock
Assumptions used to determine value of the shares purchased under the stock purchase plan using Black-Scholes option pricing model
Granted (in shares)		408,163	312,749	282,651
Number of shares
Nonvested shares at the beginning of the period (in shares)		723,176	733,591	698,885
Granted (in shares)		408,163	312,749	282,651
Vested (in shares)		(215,856)	(284,891)	(202,012)
Cancelled (in shares)		(106,184)	(38,273)	(45,933)
Nonvested shares at the end of the period (in shares)		809,299	723,176	733,591
Weighted Average Grant-Date Fair Value
Nonvested shares at the beginning of the period (in dollars per share) | $ / shares		$ 12.70	$ 13.79	$ 14.21
Granted (in dollars per share) | $ / shares		13.57	12.00	13.43
Vested (in dollars per share) | $ / shares		13.06	14.74	14.51
Cancelled (in dollars per share) | $ / shares		13.52	12.83	13.69
Nonvested shares at the end of the period (in dollars per share) | $ / shares		$ 13.13	$ 12.70	$ 13.79
Restricted Stock | Director
Assumptions used to determine value of the shares purchased under the stock purchase plan using Black-Scholes option pricing model
Granted (in shares)		36,950	44,631	29,453
Number of shares
Granted (in shares)		36,950	44,631	29,453
Weighted Average Grant-Date Fair Value
Granted (in dollars per share) | $ / shares		$ 14.05	$ 12.10	$ 13.24
Performance Share Units
Assumptions used to determine value of the shares purchased under the stock purchase plan using Black-Scholes option pricing model
Granted (in shares)		222,583
Number of shares
Granted (in shares)		222,583
Cancelled (in shares)		(19,754)
Nonvested shares at the end of the period (in shares)		202,829
Weighted Average Grant-Date Fair Value
Granted (in dollars per share) | $ / shares		$ 13.61
Cancelled (in dollars per share) | $ / shares		13.51
Nonvested shares at the end of the period (in dollars per share) | $ / shares		$ 13.62
Group of Plans
Stock Compensation
Number of stock option plans | plan					3
Group of Plans | Employee Stock Option
Stock Compensation
Options outstanding (in shares)		76,923			76,923
Number of Options
Outstanding at the end of the period (in shares)		76,923
Long Term Incentive Plan 2010
Stock Compensation
Number of shares authorized	5,150,000
Long Term Incentive Plan 2010 | Employee Stock Option
Stock Compensation
Minimum incentive stock option exercise price, expressed as a percentage of common stock grant date market value	100.00%
Number of Options
Granted (in shares)		267,433	255,503	173,560
Non-vested stock options
Granted (in shares)		267,433	255,503	173,560
Long Term Incentive Plan 2010 | Restricted Stock
Assumptions used to determine value of the shares purchased under the stock purchase plan using Black-Scholes option pricing model
Granted (in shares)		408,163
Vesting period		3 years
Number of shares
Granted (in shares)		408,163
Weighted Average Grant-Date Fair Value
Granted (in dollars per share) | $ / shares		$ 13.57