Short-Term Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	$ 265,410	$ 284,618
Gross Unrealized Gains	89	93
Gross Unrealized Losses	(440)	(636)
Fair Value	265,059	284,075
Contractual Maturities
Under 1 Year	133,946
1 to 5 Years	131,113
Total	265,059
Corporate debt
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	170,591	171,139
Gross Unrealized Gains	73	84
Gross Unrealized Losses	(237)	(357)
Fair Value	170,427	170,866
Contractual Maturities
Under 1 Year	76,363
1 to 5 Years	94,064
Total	170,427
U.S. government and agency debt
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	47,862	53,916
Gross Unrealized Gains	0	8
Gross Unrealized Losses	(153)	(134)
Fair Value	47,709	53,790
Contractual Maturities
Under 1 Year	34,068
1 to 5 Years	13,641
Total	47,709
Municipal bonds
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	22,422	22,893
Gross Unrealized Gains	16	1
Gross Unrealized Losses	(48)	(144)
Fair Value	22,390	22,750
Contractual Maturities
Under 1 Year	7,875
1 to 5 Years	14,515
Total	22,390
Other fixed income securities
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	24,535	36,670
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(2)	(1)
Fair Value	24,533	$ 36,669
Contractual Maturities
Under 1 Year	15,640
1 to 5 Years	8,893
Total	$ 24,533