Fair Value of Financial Instruments (Details) (USD $)	9 Months Ended
	Sep. 30, 2012 item	Dec. 31, 2011	Sep. 30, 2011	Dec. 31, 2010
Fair Value of Financial Instruments
Cash and cash equivalents, cost	$ 5,967,000	$ 40,025,000	$ 5,781,000	$ 14,340,000
Cash and cash equivalents, fair value	5,967,000	40,025,000
Estimated fair values of financial instruments
Cost	39,271,000	17,949,000
Unrealized Gains	27,000	6,000
Unrealized Losses	(1,000)	(5,000)
Fair Value	39,297,000	17,950,000
Fair value of available-for-sale securities with unrealized losses	7,600,000
Number of maturity groups	2
Investments by rolling maturity, Cost
Due within 1 year	30,187,000	9,412,000
Due in 1-2 years	9,084,000	8,537,000
Total	39,271,000	17,949,000
Investments by rolling maturity, Unrealized Gains
Due within 1 year	22,000	3,000
Due in 1-2 years	5,000	3,000
Total	27,000	6,000
Investments by rolling maturity, Unrealized Losses
Due within 1 year	(1,000)	(2,000)
Due in 1-2 years		(3,000)
Total	(1,000)	(5,000)
Investments by rolling maturity, Fair Value
Due within 1 year	30,208,000	9,413,000
Due in 1-2 years	9,089,000	8,537,000
Fair Value	39,297,000	17,950,000
Short-term investments
Estimated fair values of financial instruments
Cost	30,187,000	9,412,000
Unrealized Gains	22,000	3,000
Unrealized Losses	(1,000)	(2,000)
Fair Value	30,208,000	9,413,000
Investments by rolling maturity, Cost
Total	30,187,000	9,412,000
Investments by rolling maturity, Unrealized Gains
Total	22,000	3,000
Investments by rolling maturity, Unrealized Losses
Total	(1,000)	(2,000)
Investments by rolling maturity, Fair Value
Fair Value	30,208,000	9,413,000
Short-term investments | U.S. government debt securities
Estimated fair values of financial instruments
Cost	10,628,000	4,834,000
Unrealized Gains	4,000	2,000
Fair Value	10,632,000	4,836,000
Investments by rolling maturity, Cost
Total	10,628,000	4,834,000
Investments by rolling maturity, Unrealized Gains
Total	4,000	2,000
Investments by rolling maturity, Fair Value
Fair Value	10,632,000	4,836,000
Short-term investments | Corporate notes and commercial paper
Estimated fair values of financial instruments
Cost	14,403,000
Unrealized Gains	15,000
Unrealized Losses	(1,000)
Fair Value	14,417,000
Investments by rolling maturity, Cost
Total	14,403,000
Investments by rolling maturity, Unrealized Gains
Total	15,000
Investments by rolling maturity, Unrealized Losses
Total	(1,000)
Investments by rolling maturity, Fair Value
Fair Value	14,417,000
Short-term investments | Corporate notes
Estimated fair values of financial instruments
Cost		4,578,000
Unrealized Gains		1,000
Unrealized Losses		(2,000)
Fair Value		4,577,000
Investments by rolling maturity, Cost
Total		4,578,000
Investments by rolling maturity, Unrealized Gains
Total		1,000
Investments by rolling maturity, Unrealized Losses
Total		(2,000)
Investments by rolling maturity, Fair Value
Fair Value		4,577,000
Short-term investments | Certificates of deposit
Estimated fair values of financial instruments
Cost	5,156,000
Unrealized Gains	3,000
Fair Value	5,159,000
Investments by rolling maturity, Cost
Total	5,156,000
Investments by rolling maturity, Unrealized Gains
Total	3,000
Investments by rolling maturity, Fair Value
Fair Value	5,159,000
Long-term investments
Estimated fair values of financial instruments
Cost	9,084,000	8,537,000
Unrealized Gains	5,000	3,000
Unrealized Losses		(3,000)
Fair Value	9,089,000	8,537,000
Investments by rolling maturity, Cost
Total	9,084,000	8,537,000
Investments by rolling maturity, Unrealized Gains
Total	5,000	3,000
Investments by rolling maturity, Unrealized Losses
Total		(3,000)
Investments by rolling maturity, Fair Value
Fair Value	9,089,000	8,537,000
Long-term investments | U.S. government debt securities
Estimated fair values of financial instruments
Cost	6,770,000	5,721,000
Unrealized Gains	2,000	1,000
Unrealized Losses		(1,000)
Fair Value	6,772,000	5,721,000
Investments by rolling maturity, Cost
Total	6,770,000	5,721,000
Investments by rolling maturity, Unrealized Gains
Total	2,000	1,000
Investments by rolling maturity, Unrealized Losses
Total		(1,000)
Investments by rolling maturity, Fair Value
Fair Value	6,772,000	5,721,000
Long-term investments | Corporate notes
Estimated fair values of financial instruments
Cost	2,074,000	2,816,000
Unrealized Gains	2,000	2,000
Unrealized Losses		(2,000)
Fair Value	2,076,000	2,816,000
Investments by rolling maturity, Cost
Total	2,074,000	2,816,000
Investments by rolling maturity, Unrealized Gains
Total	2,000	2,000
Investments by rolling maturity, Unrealized Losses
Total		(2,000)
Investments by rolling maturity, Fair Value
Fair Value	2,076,000	2,816,000
Long-term investments | Certificates of deposit
Estimated fair values of financial instruments
Cost	240,000
Unrealized Gains	1,000
Fair Value	241,000
Investments by rolling maturity, Cost
Total	240,000
Investments by rolling maturity, Unrealized Gains
Total	1,000
Investments by rolling maturity, Fair Value
Fair Value	$ 241,000