Investments in Marketable Securities (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 114,809,000	$ 151,232,000
Net unrealized holding gains	1,000	4,000
Net unrealized holding losses	(1,000)	(2,000)
Fair Value	114,809,000	151,234,000
Maximum Term Of Available For Sale Securities	1	1
Marketable Securities, Realized Loss, Other than Temporary Impairments, Amount	0	0	0
Cash Equivalents [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	24,010,000
Net unrealized holding gains	0
Net unrealized holding losses	0
Fair Value	24,010,000
Cash Equivalents [Member] | Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	40,793,000	114,217,000
Net unrealized holding gains	0	0
Net unrealized holding losses	0	0
Fair Value	40,793,000	114,217,000
Cash Equivalents [Member] | US Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		5,500,000
Net unrealized holding gains		0
Net unrealized holding losses		0
Fair Value		5,500,000
Cash Equivalents [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	4,000,000
Net unrealized holding gains	0
Net unrealized holding losses	0
Fair Value	4,000,000
Cash Equivalents [Member] | Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		325,000
Net unrealized holding gains		0
Net unrealized holding losses		0
Fair Value		325,000
Short Term Marketable Securities [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	12,704,000
Net unrealized holding gains	1,000
Net unrealized holding losses	0
Fair Value	12,705,000
Short Term Marketable Securities [Member] | Municipal Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	9,295,000
Net unrealized holding gains	0
Net unrealized holding losses	0
Fair Value	9,295,000
Short Term Marketable Securities [Member] | US Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	7,501,000	3,115,000
Net unrealized holding gains	0	0
Net unrealized holding losses	0	(2,000)
Fair Value	7,501,000	3,113,000
Short Term Marketable Securities [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	16,506,000	26,366,000
Net unrealized holding gains	0	4,000
Net unrealized holding losses	(1,000)	0
Fair Value	16,505,000	26,370,000
Short Term Marketable Securities [Member] | Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		1,709,000
Net unrealized holding gains		0
Net unrealized holding losses		0
Fair Value		$ 1,709,000