Marketable Securities - Carrying Amounts of Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 17,607	$ 25,504	$ 27,950
Long-term marketable securities	70,646	76,563	74,140
Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	17,607	25,504	27,950
Long-term marketable securities	70,646	76,563	74,140
Total marketable securities	88,253	102,067	102,090
U.S. Government and agency obligations [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	7,625	10,511	12,967
Long-term marketable securities	70,646	76,563	74,140
Commercial paper [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	$ 9,982	$ 14,993	$ 14,983