Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Amortized Cost
Amortized cost	$ 129,121	$ 58,909
Gross Unrealized Gains
Gross Unrealized Gains	86	164
Gross Unrealized Losses
Gross Unrealized Losses	124	8
Fair Value
Fair Value	129,083	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	34,189	15,566
Maturing after one year through three years	34,030	19,797
Amortized cost	68,219	35,363
Gross Unrealized Gains
Maturing in one year or less	15	28
Maturing after one year through three years	61	99
Gross Unrealized Gains	76	127
Gross Unrealized Losses
Maturing in one year or less	5
Maturing after one year through three years	22	1
Gross Unrealized Losses	27	1
Fair Value
Maturing in one year or less	34,199	15,594
Maturing after one year through three years	34,069	19,895
Fair Value	68,268	35,489
Corporate debt securities
Amortized Cost
Maturing in one year or less	33,336	17,353
Maturing after one year through three years	27,566	6,193
Amortized cost	60,902	23,546
Gross Unrealized Gains
Maturing in one year or less	10	23
Maturing after one year through three years		14
Gross Unrealized Gains	10	37
Gross Unrealized Losses
Maturing in one year or less	32	4
Maturing after one year through three years	65	3
Gross Unrealized Losses	97	7
Fair Value
Maturing in one year or less	33,314	17,372
Maturing after one year through three years	27,501	6,204
Fair Value	$ 60,815	$ 23,576