Marketable Securities - Held to Maturity Securities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)
Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Short-term HTM securities	$ 449,535
Long-term HTM securities	187,299
Debt Securities [Member] | US Treasury Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Short-term HTM securities	150,623
Long-term HTM securities	14,792
Debt Securities [Member] | Corporate Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Short-term HTM securities	150,599
Long-term HTM securities	61,775
Debt Securities [Member] | US Government Agencies Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Short-term HTM securities	64,135
Long-term HTM securities	110,732
Debt Securities [Member] | Commercial Paper [Member]
Schedule of Held-to-maturity Securities [Line Items]
Short-term HTM securities	$ 84,178
Minimum [Member] | Short-term Marketable Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Remaining maturities on short-term and long-term marketable securities	1 month
Minimum [Member] | Long-term Marketable Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Remaining maturities on short-term and long-term marketable securities	13 months
Maximum [Member] | Short-term Marketable Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Remaining maturities on short-term and long-term marketable securities	12 months
Maximum [Member] | Long-term Marketable Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Remaining maturities on short-term and long-term marketable securities	35 months