Marketable Securities and Fair Value Disclosures (Details) (Textuals) (USD $)	6 Months Ended		12 Months Ended				6 Months Ended	12 Months Ended		1 Months Ended	12 Months Ended	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	Sep. 30, 2010	Mar. 31, 2010	Dec. 31, 2009	Jun. 30, 2010 Auction rate securities [Member]	Dec. 31, 2010 Auction rate securities [Member]	Jun. 30, 2011 Auction rate securities [Member]	Jan. 31, 2010 Mutual funds [Member]	Dec. 31, 2009 Mutual funds [Member]	Jun. 30, 2011 Maximum [Member]	Jun. 30, 2011 Minimum [Member]
Marketable Securities Fair Value Disclosures (Textuals) [Abstract]
Available-for-Sale Securities, Estimated fair value	$ 10,410,000		$ 10,130,000
Amortized cost of auction rate securities classified as trading securities sold							9,600,000	6,900,000
Excess of amortized cost over fair value of auction rate securities classified as available-for-sale securities								131,000	110,000
Loss from change in valuation of available-for-sale securities recorded in accumulated other comprehensive loss	21,000
Amortized cost of Investment in mutual fund	10,525,000		10,282,000								1,000,000
Cash (paid) received on financial instruments, net	(192,000)	932,000
Impairment of investment in mutual fund											200,000
Proceeds from sale and maturity of mutual fund										800,000
Minimum contractual maturity of ARS	90 days
Contractual maturity of ARS	10 years
Transfers of assets and liabilities between Level 1 and Level 2 of fair value measurement hierarchy	0
Period at which investors can hold the investments at par or sell the securities at auction												35 days	7 days
Cash and cash equivalents	228,358,000	161,269,000	219,448,000	189,928,000	142,067,000	195,678,000
Time deposits	62,500,000		65,600,000
Money market fund deposits	$ 26,900,000		$ 27,700,000
Maturity of time deposits less than or equal to	less than or equal to 90 days		less than or equal to 90 days