Marketable Securities - Summary of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized gains	$ 3
Cost
Debt Securities, Available-for-sale [Line Items]
Total Available-for-sale Securities	60,457	$ 64,138
Cost | Certificates of deposit | Level 1
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized losses for less than 12 months	0	250
Marketable Securities with unrecognized losses for more than 12 months	4,012	3,861
Marketable Securities with unrecognized gains	0	0
Total Available-for-sale Securities	4,012	4,111
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	4,064	3,112
Marketable Securities with unrecognized losses for more than 12 months	19,472	23,564
Marketable Securities with unrecognized gains	2	3
Total Available-for-sale Securities	23,538	26,679
Cost | Corporate notes | Level 2
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized losses for less than 12 months	14,780	13,696
Marketable Securities with unrecognized losses for more than 12 months	9,383	12,542
Marketable Securities with unrecognized gains	1,795	0
Total Available-for-sale Securities	25,958	26,238
Cost | Municipal notes | Level 2
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized losses for less than 12 months	0	2,994
Marketable Securities with unrecognized losses for more than 12 months	6,949	4,116
Marketable Securities with unrecognized gains	0	0
Total Available-for-sale Securities	6,949	7,110
Fair Value
Debt Securities, Available-for-sale [Line Items]
Total Available-for-sale Securities	60,270	63,749
Fair Value | Certificates of deposit | Level 1
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized losses for less than 12 months	0	248
Marketable Securities with unrecognized losses for more than 12 months	3,991	3,812
Marketable Securities with unrecognized gains	0	0
Total Available-for-sale Securities	3,991	4,060
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	4,058	3,105
Marketable Securities with unrecognized losses for more than 12 months	19,391	23,415
Marketable Securities with unrecognized gains	3	4
Total Available-for-sale Securities	23,452	26,524
Fair Value | Corporate notes | Level 2
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized losses for less than 12 months	14,760	13,665
Marketable Securities with unrecognized losses for more than 12 months	9,334	12,431
Marketable Securities with unrecognized gains	1,797	0
Total Available-for-sale Securities	25,891	26,096
Fair Value | Municipal notes | Level 2
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized losses for less than 12 months	0	2,982
Marketable Securities with unrecognized losses for more than 12 months	6,936	4,087
Marketable Securities with unrecognized gains	0	0
Total Available-for-sale Securities	$ 6,936	$ 7,069