Fair Value Measurements (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011
Fair Value Measurements [Abstract]
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate losses	$ 1,700,000
Available-for-sale securities [Line Items]
Auction rate securities, gross realized loss	8,800,000
Other than temporary impairment losses, investments, portion in other comprehensive income (loss), before tax	2,600,000
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities	4,700,000
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	111,000	453,000
Period which fair value of auction rate securities has been below cost	one year
Long-term Debt, Fair Value	93,500,000	93,500,000
Short-term Investments [Member]
Available-for-sale securities [Line Items]
Amortized Cost	232,392,000	229,629,000
Unrealized Gains	131,000	31,000
Unrealized Losses	(2,707,000)	(4,888,000)
Fair Value	229,816,000	224,772,000
Corporate bonds [Member] | Short-term Investments [Member]
Available-for-sale securities [Line Items]
Amortized Cost	9,598,000	8,851,000
Unrealized Gains	9,000	1,000
Unrealized Losses	(3,000)	(7,000)
Fair Value	9,604,000	8,845,000
Corporate bonds [Member] | Long-term Investments [Member]
Available-for-sale securities [Line Items]
Amortized Cost	6,351,000
Unrealized Gains	14,000
Unrealized Losses	(10,000)
Fair Value	6,355,000
Investment funds - debt securities [Member] | Short-term Investments [Member]
Available-for-sale securities [Line Items]
Amortized Cost	222,794,000	220,778,000
Unrealized Gains	122,000	30,000
Unrealized Losses	(2,704,000)	(4,881,000)
Fair Value	220,212,000	215,927,000
Auction Rate Securities [Member]
Available-for-sale securities [Line Items]
Fair Value	$ 15,500,000	$ 15,000,000