Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Amortized Cost
Amortized cost	$ 121,862	$ 58,909
Gross Unrealized Gains
Gross Unrealized Gains	117	164
Gross Unrealized Losses
Gross Unrealized Losses	(17)	(8)
Fair Value
Fair Value	121,962	59,065
Marketable securities considered to be other-than-temporarily impaired	0
U.S. government and municipal obligations
Amortized Cost
Maturing in one year or less	50,900	15,566
Maturing after one year through three years	14,104	19,797
Amortized cost	65,004	35,363
Gross Unrealized Gains
Maturing in one year or less	32	28
Maturing after one year through three years	63	99
Gross Unrealized Gains	95	127
Gross Unrealized Losses
Maturing in one year or less	(3)
Maturing after one year through three years	(2)	(1)
Gross Unrealized Losses	(5)	(1)
Fair Value
Maturing in one year or less	50,929	15,594
Maturing after one year through three years	14,165	19,895
Fair Value	65,094	35,489
Corporate debt securities
Amortized Cost
Maturing in one year or less	36,001	17,353
Maturing after one year through three years	20,857	6,193
Amortized cost	56,858	23,546
Gross Unrealized Gains
Maturing in one year or less	13	23
Maturing after one year through three years	9	14
Gross Unrealized Gains	22	37
Gross Unrealized Losses
Maturing in one year or less	(4)	(4)
Maturing after one year through three years	(8)	(3)
Gross Unrealized Losses	(12)	(7)
Fair Value
Maturing in one year or less	36,010	17,372
Maturing after one year through three years	20,858	6,204
Fair Value	$ 56,868	$ 23,576