Marketable Securities - Available for Sale (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Net unrealized loss in other comprehensive income	$ 3,300,000	$ 2,500,000
OTTI loss recognized	0
Short-term AFS securities
Amortized cost, short-term AFS securities	349,519,000	309,029,000
Gross unrealized gains, short-term AFS	2,000
Gross unrealized losses, short-term AFS	(1,299,000)	(440,000)
Fair value, short-term AFS securities	348,222,000	308,589,000
Long-term AFS securities
Amortized cost, long-term AFS securities	242,086,000	243,516,000
Gross unrealized gains, long-term AFS	14,000	0
Gross unrealized losses, long-term AFS	(2,045,000)	(2,105,000)
Fair value, long-term AFS securities	240,055,000	241,411,000
Corporate debt securities
Short-term AFS securities
Amortized cost, short-term AFS securities	138,983,000	150,334,000
Gross unrealized gains, short-term AFS	2,000
Gross unrealized losses, short-term AFS	(350,000)	(227,000)
Fair value, short-term AFS securities	138,635,000	150,107,000
Long-term AFS securities
Amortized cost, long-term AFS securities	139,179,000	92,687,000
Gross unrealized gains, long-term AFS	14,000	0
Gross unrealized losses, long-term AFS	(959,000)	(683,000)
Fair value, long-term AFS securities	138,234,000	92,004,000
U.S. Treasury securities
Short-term AFS securities
Amortized cost, short-term AFS securities	113,016,000	70,985,000
Gross unrealized gains, short-term AFS	0
Gross unrealized losses, short-term AFS	(232,000)	(34,000)
Fair value, short-term AFS securities	112,784,000	70,951,000
Long-term AFS securities
Amortized cost, long-term AFS securities	54,560,000	53,031,000
Gross unrealized gains, long-term AFS	0	0
Gross unrealized losses, long-term AFS	(628,000)	(403,000)
Fair value, long-term AFS securities	53,932,000	52,628,000
U.S. governmental agency securities
Short-term AFS securities
Amortized cost, short-term AFS securities	77,872,000	45,819,000
Gross unrealized gains, short-term AFS	0
Gross unrealized losses, short-term AFS	(717,000)	(179,000)
Fair value, short-term AFS securities	77,155,000	45,640,000
Long-term AFS securities
Amortized cost, long-term AFS securities	48,347,000	97,798,000
Gross unrealized gains, long-term AFS	0	0
Gross unrealized losses, long-term AFS	(458,000)	(1,019,000)
Fair value, long-term AFS securities	47,889,000	96,779,000
Commercial paper
Short-term AFS securities
Amortized cost, short-term AFS securities	11,800,000	32,591,000
Gross unrealized gains, short-term AFS	0	0
Gross unrealized losses, short-term AFS	0	0
Fair value, short-term AFS securities	11,800,000	32,591,000
Certificates of deposit
Short-term AFS securities
Amortized cost, short-term AFS securities	7,848,000	9,300,000
Gross unrealized gains, short-term AFS	0
Gross unrealized losses, short-term AFS	0	0
Fair value, short-term AFS securities	$ 7,848,000	$ 9,300,000