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Marketable Securities - Summary of Available-for-sale Securities (Details) - USD ($)
$ in Thousands
Jun. 30, 2015
Dec. 31, 2014
Summary of available-for-sale securities    
Available-for-sale securities, Cost $ 39,318 $ 42,112
Available-for-sale securities, Estimated Fair Value 39,289 42,140
Level 1 | Certificates of deposit    
Summary of available-for-sale securities    
Unrecognized losses for less than 12 months, Cost 3,925 2,522
Unrecognized losses for more than 12 months, Cost 620 837
Unrecognized gains, Cost 3,789 5,379
Available-for-sale securities, Cost 8,334 8,738
Unrecognized losses for less than 12 months, Estimated Fair value 3,899 2,492
Unrecognized losses for more than 12 months, Estimated Fair value 619 832
Unrecognized gains, Estimated Fair value 3,798 5,395
Available-for-sale securities, Estimated Fair Value 8,316 8,719
Level 2 | U.S. Treasury and agency notes    
Summary of available-for-sale securities    
Unrecognized losses for less than 12 months, Cost 723 1,919
Unrecognized losses for more than 12 months, Cost 200 702
Unrecognized gains, Cost 1,764 1,182
Available-for-sale securities, Cost 2,687 3,803
Unrecognized losses for less than 12 months, Estimated Fair value 720 1,910
Unrecognized losses for more than 12 months, Estimated Fair value 200 700
Unrecognized gains, Estimated Fair value 1,790 1,207
Available-for-sale securities, Estimated Fair Value 2,710 3,817
Level 2 | Corporate notes    
Summary of available-for-sale securities    
Unrecognized losses for less than 12 months, Cost 8,049 3,872
Unrecognized losses for more than 12 months, Cost 2,520 4,423
Unrecognized gains, Cost 12,893 16,897
Available-for-sale securities, Cost 23,462 25,192
Unrecognized losses for less than 12 months, Estimated Fair value 7,996 3,841
Unrecognized losses for more than 12 months, Estimated Fair value 2,507 4,405
Unrecognized gains, Estimated Fair value 12,934 16,963
Available-for-sale securities, Estimated Fair Value 23,437 25,209
Level 2 | Municipal notes    
Summary of available-for-sale securities    
Unrecognized losses for less than 12 months, Cost 1,532 739
Unrecognized losses for more than 12 months, Cost 641 457
Unrecognized gains, Cost 2,662 3,183
Available-for-sale securities, Cost 4,835 4,379
Unrecognized losses for less than 12 months, Estimated Fair value 1,518 733
Unrecognized losses for more than 12 months, Estimated Fair value 634 456
Unrecognized gains, Estimated Fair value 2,674 3,206
Available-for-sale securities, Estimated Fair Value $ 4,826 $ 4,395