Cash Equivalents and Marketable Securities (Details) (USD $)	12 Months Ended
	Dec. 31, 2012 M	Dec. 31, 2011	Dec. 31, 2010
Cash Equivalents and Marketable Securities
Maturity period for long-term debt classified as available-for-sale (in months)	12
Summary of marketable securities
Amortized Cost	$ 172,766,000	$ 70,838,000
Gross Unrealized Gains	26,000	15,000
Gross Unrealized Losses Less than 12 Months	(48,000)	(28,000)
Aggregate Fair Value	172,744,000	70,825,000
Amortized Cost
Matures in less than one year	111,184,000
Matures in one to three years	61,582,000
Total	172,766,000
Aggregate Fair Value
Matures in less than one year	111,192,000
Matures in one to three years	61,552,000
Aggregate Fair Value	172,744,000	70,825,000
Fair Value
Less than 12 months	94,406,000
Total	94,406,000
Unrealized losses
Gross Unrealized Losses Less than 12 Months	(48,000)	(28,000)
Total	(48,000)
Realized gains related to marketable securities	16,000	0	0
Corporate debt securities
Summary of marketable securities
Amortized Cost	110,540,000	52,606,000
Gross Unrealized Gains	13,000	10,000
Gross Unrealized Losses Less than 12 Months	(45,000)	(11,000)
Aggregate Fair Value	110,508,000	52,605,000
Aggregate Fair Value
Aggregate Fair Value	110,508,000	52,605,000
Fair Value
Less than 12 months	70,191,000
Total	70,191,000
Unrealized losses
Gross Unrealized Losses Less than 12 Months	(45,000)	(11,000)
Total	(45,000)
US government debt securities
Summary of marketable securities
Amortized Cost	59,811,000	11,318,000
Gross Unrealized Gains	7,000	5,000
Gross Unrealized Losses Less than 12 Months	(3,000)
Aggregate Fair Value	59,815,000	11,323,000
Aggregate Fair Value
Aggregate Fair Value	59,815,000	11,323,000
Fair Value
Less than 12 months	24,215,000
Total	24,215,000
Unrealized losses
Gross Unrealized Losses Less than 12 Months	(3,000)
Total	(3,000)
Foreign government debt securities
Summary of marketable securities
Amortized Cost	2,415,000	6,914,000
Gross Unrealized Gains	6,000
Gross Unrealized Losses Less than 12 Months		(17,000)
Aggregate Fair Value	2,421,000	6,897,000
Aggregate Fair Value
Aggregate Fair Value	2,421,000	6,897,000
Unrealized losses
Gross Unrealized Losses Less than 12 Months		$ (17,000)