Marketable Securities - Summary of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Cost
Debt Securities, Available-for-sale [Line Items]
Total Available-for-sale Securities	$ 69,902	$ 71,066
Cost | Certificates of deposit | Level 1
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized losses for less than 12 months	5,237	6,238
Marketable Securities with unrecognized losses for more than 12 months	1,087	102
Marketable Securities with unrecognized gains	0	2,088
Available-for-sale securities, Cost	6,324	8,428
Cost | U.S. Treasury and agency notes | Level 2
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized losses for less than 12 months	29,766	29,741
Marketable Securities with unrecognized losses for more than 12 months	234	137
Marketable Securities with unrecognized gains	3	152
Available-for-sale securities, Cost	30,003	30,030
Cost | Corporate notes | Level 2
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized losses for less than 12 months	21,453	18,230
Marketable Securities with unrecognized losses for more than 12 months	3,063	2,804
Marketable Securities with unrecognized gains	1,125	0
Available-for-sale securities, Cost	25,641	21,034
Cost | Municipal notes | Level 2
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized losses for less than 12 months	6,770	10,298
Marketable Securities with unrecognized losses for more than 12 months	1,074	999
Marketable Securities with unrecognized gains	90	277
Available-for-sale securities, Cost	7,934	11,574
Fair Value
Debt Securities, Available-for-sale [Line Items]
Total Available-for-sale Securities	69,381	70,868
Fair Value | Certificates of deposit | Level 1
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized losses for less than 12 months	5,183	6,222
Marketable Securities with unrecognized losses for more than 12 months	1,085	100
Marketable Securities with unrecognized gains	0	2,089
Available-for-sale securities, Cost	6,268	8,411
Fair Value | U.S. Treasury and agency notes | Level 2
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized losses for less than 12 months	29,528	29,669
Marketable Securities with unrecognized losses for more than 12 months	233	135
Marketable Securities with unrecognized gains	4	153
Available-for-sale securities, Cost	29,765	29,957
Fair Value | Corporate notes | Level 2
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized losses for less than 12 months	21,304	18,159
Marketable Securities with unrecognized losses for more than 12 months	3,047	2,788
Marketable Securities with unrecognized gains	1,125	0
Available-for-sale securities, Cost	25,476	20,947
Fair Value | Municipal notes | Level 2
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized losses for less than 12 months	6,717	10,288
Marketable Securities with unrecognized losses for more than 12 months	1,065	987
Marketable Securities with unrecognized gains	90	278
Available-for-sale securities, Cost	$ 7,872	$ 11,553