Fair Value of Financial Instruments - Contractual Maturity (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Mature within one year	$ 49,132	$ 69,525
After one year through two years	28,432	30,107
Over two years	9,464	5,501
Fair value	87,028	105,133
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Mature within one year	12,194	47,488
After one year through two years	0	0
Over two years	0	0
Fair value	12,194	47,488
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Mature within one year		1,128
After one year through two years		0
Over two years		0
Fair value		1,128
US government agencies
Schedule of Available-for-sale Securities [Line Items]
Mature within one year	23,721	4,004
After one year through two years	17,827	14,833
Over two years	0	0
Fair value	41,548	18,837
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Mature within one year	13,217	16,905
After one year through two years	1,799	15,274
Over two years	0	0
Fair value	15,016	32,179
Asset backed securities
Schedule of Available-for-sale Securities [Line Items]
Mature within one year	0	0
After one year through two years	8,806	0
Over two years	9,464	5,501
Fair value	$ 18,270	$ 5,501