Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Amortized Cost
Amortized cost	$ 106,648	$ 58,909
Gross Unrealized Gains
Gross Unrealized Gains	144	164
Gross Unrealized Losses
Gross Unrealized Losses	42	8
Fair Value
Fair Value	106,750	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	21,016	15,566
Maturing after one year through three years	17,470	19,797
Amortized cost	38,486	35,363
Gross Unrealized Gains
Maturing in one year or less	30	28
Maturing after one year through three years	84	99
Gross Unrealized Gains	114	127
Gross Unrealized Losses
Maturing after one year through three years	1	1
Gross Unrealized Losses	1	1
Fair Value
Maturing in one year or less	21,046	15,594
Maturing after one year through three years	17,553	19,895
Fair Value	38,599	35,489
Corporate debt securities
Amortized Cost
Maturing in one year or less	23,909	17,353
Maturing after one year through three years	44,253	6,193
Amortized cost	68,162	23,546
Gross Unrealized Gains
Maturing in one year or less	24	23
Maturing after one year through three years	6	14
Gross Unrealized Gains	30	37
Gross Unrealized Losses
Maturing in one year or less	10	4
Maturing after one year through three years	31	3
Gross Unrealized Losses	41	7
Fair Value
Maturing in one year or less	23,923	17,372
Maturing after one year through three years	44,228	6,204
Fair Value	$ 68,151	$ 23,576