Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Summary of valuation of investments and financial instruments pricing levels
Total	$ 63,584	$ 67,973
(Level 1) [Member]
Summary of valuation of investments and financial instruments pricing levels
Total	27,271	32,503
(Level 2) [Member]
Summary of valuation of investments and financial instruments pricing levels
Total	36,313	35,470
Investments [Member]
Summary of valuation of investments and financial instruments pricing levels
Total	60,705	65,286
Investments [Member] \| (Level 1) [Member]
Summary of valuation of investments and financial instruments pricing levels
Total	24,392	29,816
Investments [Member] \| (Level 2) [Member]
Summary of valuation of investments and financial instruments pricing levels
Total	36,313	35,470
Investments [Member] \| Money Market Mutual Funds [Member]
Summary of valuation of investments and financial instruments pricing levels
Total	24,392	29,816
Investments [Member] \| Money Market Mutual Funds [Member] \| (Level 1) [Member]
Summary of valuation of investments and financial instruments pricing levels
Total	24,392	29,816
Investments [Member] \| Commercial Paper [Member]
Summary of valuation of investments and financial instruments pricing levels
Total	7,494	5,493
Investments [Member] \| Commercial Paper [Member] \| (Level 2) [Member]
Summary of valuation of investments and financial instruments pricing levels
Total		5,493
Investments [Member] \| Corporate bonds and notes [Member]
Summary of valuation of investments and financial instruments pricing levels
Total	28,819	29,977
Investments [Member] \| Corporate bonds and notes [Member] \| (Level 2) [Member]
Summary of valuation of investments and financial instruments pricing levels
Total	28,819	29,977
Deferred compensation plan assets [Member]
Summary of valuation of investments and financial instruments pricing levels
Total	2,879	2,687
Deferred compensation plan assets [Member] \| (Level 1) [Member]
Summary of valuation of investments and financial instruments pricing levels
Total	$ 2,879	$ 2,687

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

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