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Marketable Securities - Summary of Available-for-sale Securities (Details) - USD ($)
$ in Thousands
Mar. 31, 2016
Dec. 31, 2015
Summary of available-for-sale securities    
Available-for-sale securities, Cost $ 32,691 $ 32,975
Available-for-sale securities, Estimated Fair Value 32,719 32,815
Level 1 | Certificates of deposit    
Summary of available-for-sale securities    
Unrecognized losses for less than 12 months, Cost 465 4,810
Unrecognized losses for more than 12 months, Cost 0 239
Unrecognized gains, Cost 6,481 2,800
Available-for-sale securities, Cost 6,946 7,849
Unrecognized losses for less than 12 months, Estimated Fair value 464 4,797
Unrecognized losses for more than 12 months, Estimated Fair value 0 238
Unrecognized gains, Estimated Fair value 6,510 2,805
Available-for-sale securities, Estimated Fair Value 6,974 7,840
Level 2 | U.S. Treasury and agency notes    
Summary of available-for-sale securities    
Unrecognized losses for less than 12 months, Cost 0 860
Unrecognized losses for more than 12 months, Cost 0 0
Unrecognized gains, Cost 2,028 736
Available-for-sale securities, Cost 2,028 1,596
Unrecognized losses for less than 12 months, Estimated Fair value 0 857
Unrecognized losses for more than 12 months, Estimated Fair value 0 0
Unrecognized gains, Estimated Fair value 2,037 738
Available-for-sale securities, Estimated Fair Value 2,037 1,595
Level 2 | Corporate notes    
Summary of available-for-sale securities    
Unrecognized losses for less than 12 months, Cost 5,084 14,638
Unrecognized losses for more than 12 months, Cost 3,160 2,080
Unrecognized gains, Cost 11,949 3,334
Available-for-sale securities, Cost 20,193 20,052
Unrecognized losses for less than 12 months, Estimated Fair value 5,056 14,516
Unrecognized losses for more than 12 months, Estimated Fair value 3,139 2,061
Unrecognized gains, Estimated Fair value 11,981 3,339
Available-for-sale securities, Estimated Fair Value 20,176 19,916
Level 2 | Municipal notes    
Summary of available-for-sale securities    
Unrecognized losses for less than 12 months, Cost 666 1,742
Unrecognized losses for more than 12 months, Cost 330 301
Unrecognized gains, Cost 2,528 1,435
Available-for-sale securities, Cost 3,524 3,478
Unrecognized losses for less than 12 months, Estimated Fair value 661 1,725
Unrecognized losses for more than 12 months, Estimated Fair value 329 298
Unrecognized gains, Estimated Fair value 2,542 1,441
Available-for-sale securities, Estimated Fair Value $ 3,532 $ 3,464