Marketable Securities (Available-for-sale Securities) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Summary of available-for-sale securities
Available-for-sale securities, Cost	$ 64,784	$ 64,465
Available-for-sale securities, Estimated Fair Value	64,916	65,049
Level 1 | Certificates of deposit
Summary of available-for-sale securities
Unrecognized losses for less than 12 months	1,623	1,578
Unrecognized losses for less than 12 months, Estimated Fair value	1,615	1,571
Unrecognized losses for more than 12 months, Cost	354	508
Unrecognized losses for more than 12 months, Estimated Fair value	353	507
Unrecognized gains, Cost	6,633	5,586
Unrecognized gains, Estimated Fair value	6,667	5,628
Available-for-sale securities, Cost	8,610	7,672
Available-for-sale securities, Estimated Fair Value	8,635	7,706
Level 2 | US Treasury and agency notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months	3,913	3,057
Unrecognized losses for less than 12 months, Estimated Fair value	3,894	3,024
Unrecognized losses for more than 12 months, Cost	5,312	874
Unrecognized losses for more than 12 months, Estimated Fair value	5,288	873
Unrecognized gains, Cost	7,195	12,175
Unrecognized gains, Estimated Fair value	7,244	12,267
Available-for-sale securities, Cost	16,420	16,106
Available-for-sale securities, Estimated Fair Value	16,426	16,164
Level 2 | Corporate notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months	9,479	1,993
Unrecognized losses for less than 12 months, Estimated Fair value	9,305	1,971
Unrecognized losses for more than 12 months, Cost	329	201
Unrecognized losses for more than 12 months, Estimated Fair value	328	200
Unrecognized gains, Cost	20,423	29,210
Unrecognized gains, Estimated Fair value	20,668	29,653
Available-for-sale securities, Cost	30,231	31,404
Available-for-sale securities, Estimated Fair Value	30,301	31,824
Level 2 | Municipal notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months	2,185	1,961
Unrecognized losses for less than 12 months, Estimated Fair value	2,160	1,948
Unrecognized losses for more than 12 months, Cost	720	620
Unrecognized losses for more than 12 months, Estimated Fair value	716	613
Unrecognized gains, Cost	6,618	6,702
Unrecognized gains, Estimated Fair value	6,678	6,794
Available-for-sale securities, Cost	9,523	9,283
Available-for-sale securities, Estimated Fair Value	$ 9,554	$ 9,355