Marketable Securities - Carrying Amounts of Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 25,504	$ 49,968
Long-term marketable securities	76,563	67,234
Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	25,504	49,968
Long-term marketable securities	76,563	67,234
U.S. Government and agency obligations [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	10,511	10,000
Long-term marketable securities	76,563	67,234
Commercial paper [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 14,993	$ 39,968