Cash Equivalents and Marketable Securities (Details) (USD $)	9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Cash Equivalents and Marketable Securities
Maximum maturity period for short-term marketable securities	12 months
Maturity period for long-term debt classified as available-for-sale	12 months
Marketable securities	$ 0
Summary of marketable securities
Amortized Cost		172,766,000
Gross Unrealized Gains		26,000
Gross Unrealized Losses Less than 12 Months		(48,000)
Aggregate Fair Value		172,744,000
Corporate debt securities
Summary of marketable securities
Amortized Cost		110,540,000
Gross Unrealized Gains		13,000
Gross Unrealized Losses Less than 12 Months		(45,000)
Aggregate Fair Value		110,508,000
US government debt securities
Summary of marketable securities
Amortized Cost		59,811,000
Gross Unrealized Gains		7,000
Gross Unrealized Losses Less than 12 Months		(3,000)
Aggregate Fair Value		59,815,000
Foreign government debt securities
Summary of marketable securities
Amortized Cost		2,415,000
Gross Unrealized Gains		6,000
Aggregate Fair Value		$ 2,421,000