FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS (Details) (USD $)			3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2012 Maximum Y	Mar. 31, 2012 Minimum D	Mar. 31, 2012 Fair value disclosure	Dec. 31, 2011 Fair value disclosure	Mar. 31, 2012 Fair value disclosure Cash deposits	Dec. 31, 2011 Fair value disclosure Cash deposits	Mar. 31, 2012 Fair value disclosure Variable rate demand notes	Dec. 31, 2011 Fair value disclosure Variable rate demand notes	Mar. 31, 2012 Fair value disclosure Money market funds	Dec. 31, 2011 Fair value disclosure Money market funds	Mar. 31, 2012 Fair value disclosure Certificates of deposit	Dec. 31, 2011 Fair value disclosure Certificates of deposit
Fair value disclosure
Cash and cash equivalents					$ 138,516,000	$ 141,295,000	$ 80,127,000	$ 98,258,000	$ 29,786,000	$ 29,735,000	$ 28,603,000	$ 13,302,000
Short-term investments	45,167,000	33,960,000											45,167,000	33,960,000
Restricted cash equivalents and short-term investments	29,544,000	52,693,000			29,544,000	52,693,000	4,003,000	11,842,000			5,163,000	5,375,000	20,378,000	35,476,000
Concentrations of Credit Risk of Financial Instruments
Term of original maturity of FDIC-insured unrestricted certificates of deposit classified as short-term investments (in days or years)			1	91
Cash, cash equivalents and certificates of deposit which are not FDIC-insured	$ 107,800,000	$ 98,800,000