Investments (Details) (US Treasury Securities [Member], USD $) In Thousands, unless otherwise specified	Mar. 31, 2012		Dec. 31, 2011
US Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Marketable debt securities - due within one year - Amortized cost	$ 20,005		$ 20,004
Marketable debt securities - due in one to two years - Amortized cost	10,014		10,017
Marketable debt securities - Amortized cost	30,019		30,021
Marketable debt securities - due within one year - Fair value	20,011	[1]	20,020	[1]
Marketable debt securities - due in one to two years - Fair value	10,029	[1]	10,042	[1]
Marketable debt securities - Fair value	$ 30,040	[1]	$ 30,062	[1]

[1] See Note 2 for discussion of fair value measurements.