Capital Transactions (Details) (USD $)	0 Months Ended	12 Months Ended
	May 04, 2010	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Capital Transactions
Stock based compensation expense		5,318,000	4,363,000	4,693,000
Preferred Stock
Preferred stock, shares authorized		5,000,000	5,000,000
Preferred stock outstanding (in shares)		0
Stock Compensation
Additional shares of common stock available for issuance		0
Stock Options and Restricted Stock
Compensation expenses
Total unrecognized compensation cost		4,000,000
Unrecognized compensation cost recognized over a weighted average period (in years)		1 year 2 months 12 days
Stock Option
Stock Compensation
Options outstanding (in shares)		2,888,074	3,407,575	3,653,859
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)		1.32%	1.21%	1.23%
Risk-free interest rate (as a percent)		1.50%	0.92%	0.81%
Average expected life		5 years 9 months 18 days	6 years	5 years 7 months 6 days
Expected volatility of common stock (as a percent)		0.431%	0.446%	0.409%
Forfeiture rate (as a percent)		0.00%	0.00%	0.00%
Weighted average fair value of option grants		4.47	5.04	4.43
Weighted Average Grant-Date Fair Value
Granted (in dollars per share)		4.36
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)		10 years
Number of Options
Outstanding at the beginning of the period (in shares)		3,407,575	3,653,859	4,176,673
Granted (in shares)		255,503	173,560	200,115
Exercised (in shares)		(6,701)	(75,080)	(179,204)
Cancelled (in shares)		(768,303)	(344,764)	(543,725)
Outstanding at the end of the period (in shares)		2,888,074	3,407,575	3,653,859
Exercisable (in shares)		2,324,336	2,818,464
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)		17.99	18.44	19.26
Granted (in dollars per share)		11.96	13.24	13.06
Exercised (in dollars per share)		12.10	10.91	10.57
Cancelled (in dollars per share)		6.81	20.67	25.35
Outstanding at the end of the period (in dollars per share)		16.46	17.99	18.44
Exercisable (in dollars per share)		17.39	18.83
Weighted Average Remaining Contractual Term
Outstanding, Weighted Average Remaining Contractual Term (in years)		1 year 8 months 12 days	1 year 9 months 18 days
Exercisable, Weighted Average Remaining Contractual Term (in years)		9 months 18 days	1 year 1 month 6 days
Aggregate Intrinsic Value
Total intrinsic value of options exercised		30,000,000	172,000,000	191,000,000
Non-vested stock options
Non-vested shares at beginning of year (in shares)		589,111
Granted (in shares)		255,503	173,560	200,115
Vested (in shares)		(217,918)
Cancelled (in shares)		62,958
Non-vested shares at end of year (in shares)		563,738	589,111
Weighted Average Grant-Date Fair Value
Non-vested shares at beginning of year (in dollars per share)		5.07
Weighted average fair value of option grants		4.47	5.04	4.43
Vested (in dollars per share)		5.58
Cancelled (in dollars per share)		4.74
Non-vested shares at end of year (in dollars per share)		4.56	5.07
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)		357,380	312,104	318,139
Number of shares
Nonvested shares at the beginning of the period (in shares)		733,591	698,885	611,602
Granted (in shares)		357,380	312,104	318,139
Vested (in shares)		(329,522)	(231,465)	(212,841)
Cancelled (in shares)		(38,273)	(45,933)	(18,015)
Nonvested shares at the end of the period (in shares)		723,176	733,591	698,885
Weighted Average Grant-Date Fair Value
Nonvested shares at the beginning of the period (in dollars per share)		13.79	14.21	15.08
Granted (in dollars per share)		12.01	13.41	13.04
Vested (in dollars per share)		14.38	14.35	14.95
Cancelled (in dollars per share)		12.83	13.69	14.20
Nonvested shares at the end of the period (in dollars per share)		12.70	13.79	14.21
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)		44,631
Number of shares
Granted (in shares)		44,631
Weighted Average Grant-Date Fair Value
Granted (in dollars per share)		12.10
Group of Plans
Stock Compensation
Number of stock option plans		3
Group of Plans | Stock Option
Stock Compensation
Options outstanding (in shares)		1,821,804
Number of Options
Outstanding at the end of the period (in shares)		1,821,804
2010 Incentive Plan
Stock Compensation
Number of shares authorized	5,150,000
2010 Incentive Plan | Stock Option
Stock Compensation
Minimum incentive stock option exercise price, expressed as a percentage of common stock grant date market value	100.00%
Assumptions used to determine value of the shares purchased under the stock purchase plan using Black-Scholes option pricing model
Granted (in shares)		255,503	173,560	200,115
Number of shares
Granted (in shares)		255,503	173,560	200,115
2010 Incentive Plan | 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)		312,749
Vesting period		3 years
Number of shares
Granted (in shares)		312,749
Weighted Average Grant-Date Fair Value
Granted (in dollars per share)		12.01