Marketable Securities - Summary of Available-for-sale Securities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Summary of available-for-sale securities
Available-for-sale securities, Cost	$ 55,200	$ 64,465
Available-for-sale securities, Estimated Fair Value	55,436	65,049
Level 1 | Certificates of deposit
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	1,690	1,578
Unrecognized losses for less than 12 months, Estimated Fair value	1,677	1,571
Unrecognized losses for more than 12 months, Cost	110	508
Unrecognized losses for more than 12 months, Estimated Fair value	110	507
Unrecognized gains, Cost	6,298	5,586
Unrecognized gains, Estimated Fair value	6,334	5,628
Available-for-sale securities, Cost	8,098	7,672
Available-for-sale securities, Estimated Fair Value	8,121	7,706
Level 2 | U.S. Treasury and agency notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	4,672	3,057
Unrecognized losses for less than 12 months, Estimated Fair value	4,664	3,024
Unrecognized losses for more than 12 months, Cost	1,699	874
Unrecognized losses for more than 12 months, Estimated Fair value	1,694	873
Unrecognized gains, Cost	3,713	12,175
Unrecognized gains, Estimated Fair value	3,760	12,267
Available-for-sale securities, Cost	10,084	16,106
Available-for-sale securities, Estimated Fair Value	10,118	16,164
Level 2 | Corporate notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	7,270	1,993
Unrecognized losses for less than 12 months, Estimated Fair value	7,192	1,971
Unrecognized losses for more than 12 months, Cost	530	201
Unrecognized losses for more than 12 months, Estimated Fair value	523	200
Unrecognized gains, Cost	21,945	29,210
Unrecognized gains, Estimated Fair value	22,173	29,653
Available-for-sale securities, Cost	29,745	31,404
Available-for-sale securities, Estimated Fair Value	29,888	31,824
Level 2 | Municipal notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	1,688	1,961
Unrecognized losses for less than 12 months, Estimated Fair value	1,677	1,948
Unrecognized losses for more than 12 months, Cost	318	620
Unrecognized losses for more than 12 months, Estimated Fair value	316	613
Unrecognized gains, Cost	5,267	6,702
Unrecognized gains, Estimated Fair value	5,316	6,794
Available-for-sale securities, Cost	7,273	9,283
Available-for-sale securities, Estimated Fair Value	$ 7,309	$ 9,355