Stock Options And Employee Stock Purchase Plan (Narrative) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Allocated Share-based Compensation Expense	$ 2,809,000	$ 3,004,000
Number of options outstanding	505,270		566,428
Number of options granted	0
Cash received upon the issuance of common stock	$ 58,000	61,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Total Intrinsic Value	1,800,000	100,000
Stock Options [Member]
Allocated Share-based Compensation Expense	100,000	200,000
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized	900,000		$ 1,100,000
Restricted Stock And Restricted Stock Unit (RSU) [Member]
Allocated Share-based Compensation Expense	2,700,000	$ 3,000,000
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized	$ 32,700,000		$ 34,500,000
Restricted Stock [Member]
Shares, Granted	18,000
Weighted-Average Grant-Date Fair Value, Granted	$ 63.51
Number of stocks outstanding	691,361		704,804
Restricted Stock Units (RSUs) [Member]
Shares, Granted	1,250
Number of stocks outstanding	50,920		56,245
1997 Stock Option Plan [Member]
Additional shares authorized for issuance	840,000
Maximum issuance of common stock	12,000,000
1997 Stock Option Plan [Member] | Stock Options [Member]
Number of options outstanding	56,498
1997 Stock Option Plan [Member] | Restricted Stock [Member]
Number of stocks outstanding	0
2007 Stock Plan [Member]
Number of options outstanding	386,770
Number of stocks outstanding	42,170
Maximum issuance of common stock	4,500,000
2015 Stock Option Plan [Member]
Number of options outstanding	62,000
Number of stocks outstanding	8,750
Maximum issuance of common stock	4,200,000
Employee Stock Purchase Plan [Member] | Common Stock [Member]
Market value of common stock on the date of grant for incentive stock options	95.00%
Maximum earnings withheld by the employees	15.00%
Number of shares purchased under the plan	842	1,114
Cash received upon the issuance of common stock	$ 58,000	$ 61,000
Number of shares available for issuance	1,629,602
Minimum [Member]
Market value of common stock on the date of grant for incentive stock options	85.00%