Shareholders' Equity (Details) - Restricted Stock Units (RSUs) [Member] - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restricted stock units under the 2006 and 2011 Incentive Plan [Abstract]
Maximum shares granted to employees (in shares)	700
Vesting period	5 years
Awards granted in February 2017 and after	3 years
Vesting date fair value of vested restricted stock units during the period	$ 1,869	$ 1,991	$ 3,289
Increase in tax benefit	67	290	220
Compensation expense	2,032	2,978	2,426
Income tax benefits	528	$ 1,132	934
Anticipated additional compensation costs to be recognized in future periods	$ 3,022
Maximum date compensation costs will be recognized in future periods	Dec. 31, 2022
Weighted average period over which additional compensation cost will be expensed	1 year 9 months 18 days
Restricted stock units [Roll Forward]
Unvested restricted stock units, beginning of year (in shares)	161
Units granted (in shares)	61
Units forfeited (in shares)	(25)
Units vested (in shares)	(32)
Unvested restricted stock units, end of year (in shares)	165	161
Weighted Average Grant Date Fair Value [Abstract]
Weighted average grant date fair value, beginning balance (in dollars per share)	$ 53.09
Weighted average grant date fair value, granted (in dollars per share)	58.45
Weighted average grant date fair value, forfeited (in dollars per share)	50.84
Weighted average grant date fair value, vested (in dollars per share)	45.79
Weighted average grant date fair value, ending balance (in dollars per share)	$ 56.82	$ 53.09
Grant date fair value of restricted stock units granted	$ 3,553	$ 5,399	$ 1,946