Marketable Securities - Summary of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Summary of available-for-sale securities
Available-for-sale securities, Cost	$ 71,066	$ 26,773
Available-for-sale securities, Estimated Fair Value	70,868	26,675
Level 1 | Certificates of deposit
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	6,238	1,868
Unrecognized losses for less than 12 months, Estimated Fair value	6,222	1,863
Unrecognized losses for more than 12 months, Cost	102	0
Unrecognized losses for more than 12 months, Estimated Fair value	100	0
Unrecognized gains, Cost	2,088	3,320
Unrecognized gains, Estimated Fair value	2,089	3,329
Available-for-sale securities, Cost	8,428	5,188
Available-for-sale securities, Estimated Fair Value	8,411	5,192
Level 2 | U.S. Treasury and agency notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	29,741	947
Unrecognized losses for less than 12 months, Estimated Fair value	29,669	946
Unrecognized losses for more than 12 months, Cost	137	0
Unrecognized losses for more than 12 months, Estimated Fair value	135	0
Unrecognized gains, Cost	152	857
Unrecognized gains, Estimated Fair value	153	859
Available-for-sale securities, Cost	30,030	1,804
Available-for-sale securities, Estimated Fair Value	29,957	1,805
Level 2 | Corporate notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	18,230	11,658
Unrecognized losses for less than 12 months, Estimated Fair value	18,159	11,592
Unrecognized losses for more than 12 months, Cost	2,804	1,053
Unrecognized losses for more than 12 months, Estimated Fair value	2,788	1,042
Unrecognized gains, Cost	0	3,431
Unrecognized gains, Estimated Fair value	0	3,435
Available-for-sale securities, Cost	21,034	16,142
Available-for-sale securities, Estimated Fair Value	20,947	16,069
Level 2 | Municipal notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	10,298	2,556
Unrecognized losses for less than 12 months, Estimated Fair value	10,288	2,526
Unrecognized losses for more than 12 months, Cost	999	271
Unrecognized losses for more than 12 months, Estimated Fair value	987	269
Unrecognized gains, Cost	277	812
Unrecognized gains, Estimated Fair value	278	814
Available-for-sale securities, Cost	11,574	3,639
Available-for-sale securities, Estimated Fair Value	$ 11,553	$ 3,609