Shareholders' Equity (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Shareholders protection right agreement [Abstract]
Common stock, par value (in dollars per shares)	$ 0.20	$ 0.20
Unit of series A participating preferred stock a right is entitled to purchase	one one-hundredth
Par value of series A participating preferred stock, (in dollars per share)	$ 1.00	$ 1.00
Purchase price of series A participating stock (in dollars per shares)	$ 72.00
Number of business days when rights will separate from Common Stock (in days)	10 days
Percentage of beneficial ownership of outstanding shares that will cause right to separate from common stock (in hundredths)	15.00%
Percentage of tender offer or exchange offer of outstanding shares that will cause the right to separate from common stock (in hundredths)	15.00%
Number of business days following announcement of an acquiring person when the board may terminate the rights (in days)	10 days
Current market value of common stock that each right can purchase	Twice the Unit purchase price
Restricted Stock Units (RSUs) [Member]
Restricted stock units under the 2006 and 2011 Incentive Plan [Abstract]
Award vesting period (in years)	5 years
Maximum shares granted to employees (in shares)	700
Terms for stock vesting if earlier than five years	Each award vests at the end of five years from the date of grant, or at the time a recipient retires after reaching age 65, if earlier
Fair value of vested RSU's	$ 3,045	$ 2,405	$ 2,719
Reduction in tax benefit	470	77	67
Compensation expense	961	1,231	1,054
Income tax benefits	348	417	387
Anticipated additional compensation costs to be recognized in future periods	2,217
Weighted average period over which additional compensation cost will be expensed (in years)	3 years 9 months 18 days
Grant date fair value of restricted stock units granted	$ 561	$ 763	$ 1,303
Restricted stock units [Roll Forward]
Unvested restricted stock units, beginning of year (in shares)	262
Restricted stock units granted (in shares)	14
Restricted stock units forfeited (in shares)	(4)
Restricted stock units vested (in shares)	(75)
Unvested restricted stock units, end of year (in shares)	197	262
Weighted Average Grant Date Fair Value [Abstract]
Weighted average grant date fair value, beginning balance (in dollars per share)	$ 30.54
Weighted average grant date fair value, granted (in dollars per share)	$ 40.52
Weighted average grant date fair value, forfeited (in dollars per share)	$ 32.65
Weighted average grant date fair value, vested (in dollars per share)	$ 24.38
Weighted average grant date fair value, ending balance (in dollars per share)	$ 33.54	$ 30.54