Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Summary of available-for-sale securities
Available-for-sale securities, Cost	$ 65,364	$ 64,465
Available-for-sale securities, Estimated Fair Value	65,905	65,049
Level 1 | Certificates of deposit
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	1,435	1,578
Unrecognized losses for less than 12 months, Estimated Fair value	1,431	1,571
Unrecognized losses for more than 12 months, Cost	58	508
Unrecognized losses for more than 12 months, Estimated Fair value	58	507
Unrecognized gains, Cost	7,040	5,586
Unrecognized gains, Estimated Fair value	7,102	5,628
Available-for-sale securities, Cost	8,533	7,672
Available-for-sale securities, Estimated Fair Value	8,591	7,706
Level 2 | U.S. Treasury and agency notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	992	3,057
Unrecognized losses for less than 12 months, Estimated Fair value	982	3,024
Unrecognized losses for more than 12 months, Cost	3,936	874
Unrecognized losses for more than 12 months, Estimated Fair value	3,909	873
Unrecognized gains, Cost	12,177	12,175
Unrecognized gains, Estimated Fair value	12,271	12,267
Available-for-sale securities, Cost	17,105	16,106
Available-for-sale securities, Estimated Fair Value	17,162	16,164
Level 2 | Corporate notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	5,207	1,993
Unrecognized losses for less than 12 months, Estimated Fair value	5,156	1,971
Unrecognized losses for more than 12 months, Cost	0	201
Unrecognized losses for more than 12 months, Estimated Fair value	0	200
Unrecognized gains, Cost	25,278	29,210
Unrecognized gains, Estimated Fair value	25,655	29,653
Available-for-sale securities, Cost	30,485	31,404
Available-for-sale securities, Estimated Fair Value	30,811	31,824
Level 2 | Municipal notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	1,286	1,961
Unrecognized losses for less than 12 months, Estimated Fair value	1,281	1,948
Unrecognized losses for more than 12 months, Cost	827	620
Unrecognized losses for more than 12 months, Estimated Fair value	822	613
Unrecognized gains, Cost	7,128	6,702
Unrecognized gains, Estimated Fair value	7,238	6,794
Available-for-sale securities, Cost	9,241	9,283
Available-for-sale securities, Estimated Fair Value	$ 9,341	$ 9,355