Marketable Securities (Narrative) (Details) (USD $)	3 Months Ended
	Mar. 31, 2012 Y	Dec. 31, 2011
Schedule of Trading Securities and Other Trading Assets [Line Items]
Company's portfolios average maturity term	3
Debt securities in an unrealized loss position	22	18
Average estimated fair value	$ 3,500,000	$ 2,900,000
Average unrealized loss	31,000	11,000
Marketable securities, net of current portion	88,101,000	62,237,000
Investment policy limit on the effective maturity term for individual securities	6 years
Investment policy limit on the effective maturity term for average portfolio	3 years
Deferred Compensation Plan Assets [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Non-current deferred compensation plan assets invested in mutual funds	6,600,000	6,100,000
Equity Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Marketable securities, net of current portion	11,200,000	9,200,000
Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Marketable securities, net of current portion	$ 76,900,000	$ 53,000,000
Moody's, A3 Rating [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Minimum credit rating by credit agency	A3
Standard & Poor's, A- Rating [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Minimum credit rating by credit agency	A-
Moody's, Aa1 Rating [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Company's portfolio average credit quality as defined by Moody's	AA1
Minimum [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Contractual maturity term classified as non-current marketable securities	12 months