Marketable Securities - Carrying Amounts of Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Marketable Securities [Line Items]
Held-to-maturity Securities	$ 132,790
Short-term marketable securities	67,775	$ 64,884
Long-term marketable securities	65,015	62,895
Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	67,775	64,884
Long-term marketable securities	65,015	62,895
U.S. Government and agency obligations [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	17,910	10,002
Long-term marketable securities	59,993	62,895
Commercial paper [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Short-term marketable securities	49,865	54,882
Corporate Bond Securities [Member] | Held-to-Maturity [Member]
Schedule of Marketable Securities [Line Items]
Long-term marketable securities	$ 5,022	$ 0