Marketable Securities - Summary of Available-for-sale Securities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Summary of available-for-sale securities
Available-for-sale securities, Cost	$ 42,112	$ 55,200
Available-for-sale securities, Estimated Fair Value	42,140	55,436
Level 1 | Certificates of deposit
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	2,522	1,690
Unrecognized losses for less than 12 months, Estimated Fair value	2,492	1,677
Unrecognized losses for more than 12 months, Cost	837	110
Unrecognized losses for more than 12 months, Estimated Fair value	832	110
Unrecognized gains, Cost	5,379	6,298
Unrecognized gains, Estimated Fair value	5,395	6,334
Available-for-sale securities, Cost	8,738	8,098
Available-for-sale securities, Estimated Fair Value	8,719	8,121
Level 2 | U.S. Treasury and agency notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	1,919	4,672
Unrecognized losses for less than 12 months, Estimated Fair value	1,910	4,664
Unrecognized losses for more than 12 months, Cost	702	1,699
Unrecognized losses for more than 12 months, Estimated Fair value	700	1,694
Unrecognized gains, Cost	1,182	3,713
Unrecognized gains, Estimated Fair value	1,207	3,760
Available-for-sale securities, Cost	3,803	10,084
Available-for-sale securities, Estimated Fair Value	3,817	10,118
Level 2 | Corporate notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	3,872	7,270
Unrecognized losses for less than 12 months, Estimated Fair value	3,841	7,192
Unrecognized losses for more than 12 months, Cost	4,423	530
Unrecognized losses for more than 12 months, Estimated Fair value	4,405	523
Unrecognized gains, Cost	16,897	21,945
Unrecognized gains, Estimated Fair value	16,963	22,173
Available-for-sale securities, Cost	25,192	29,745
Available-for-sale securities, Estimated Fair Value	25,209	29,888
Level 2 | Municipal notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	739	1,688
Unrecognized losses for less than 12 months, Estimated Fair value	733	1,677
Unrecognized losses for more than 12 months, Cost	457	318
Unrecognized losses for more than 12 months, Estimated Fair value	456	316
Unrecognized gains, Cost	3,183	5,267
Unrecognized gains, Estimated Fair value	3,206	5,316
Available-for-sale securities, Cost	4,379	7,273
Available-for-sale securities, Estimated Fair Value	$ 4,395	$ 7,309