Fair Value of Financial Instruments - Contractual Maturity (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Mature within one year	$ 70,346	$ 94,843
After one year through two years	20,807	0
Over two years	3,998	0
Fair value	95,151	94,843
Commercial Paper
Schedule of Available-for-sale Securities [Line Items]
Mature within one year	55,571	32,376
After one year through two years	0	0
Over two years	0	0
Fair value	55,571	32,376
US Government Agencies
Schedule of Available-for-sale Securities [Line Items]
Mature within one year	4,002	62,467
After one year through two years	4,713	0
Over two years	0	0
Fair value	8,715	62,467
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Mature within one year	1,126
After one year through two years	0
Over two years	0
Fair value	1,126
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Mature within one year	9,647
After one year through two years	16,094
Over two years	0
Fair value	25,741
Asset backed securities
Schedule of Available-for-sale Securities [Line Items]
Mature within one year	0
After one year through two years	0
Over two years	3,998
Fair value	$ 3,998