Marketable Securities (Available-for-sale Securities) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Summary of available-for-sale securities
Available-for-sale securities, Cost	$ 64,257	$ 64,465
Available-for-sale securities, Estimated Fair Value	64,487	65,049
Level 1 | Certificates of deposit
Summary of available-for-sale securities
Unrecognized losses for less than 12 months	1,717	1,578
Unrecognized losses for less than 12 months, Estimated Fair value	1,707	1,571
Unrecognized losses for more than 12 months, Cost	111	508
Unrecognized losses for more than 12 months, Estimated Fair value	111	507
Unrecognized gains, Cost	7,006	5,586
Unrecognized gains, Estimated Fair value	7,050	5,628
Available-for-sale securities, Cost	8,834	7,672
Available-for-sale securities, Estimated Fair Value	8,868	7,706
Level 2 | US Treasury and agency notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months	5,853	3,057
Unrecognized losses for less than 12 months, Estimated Fair value	5,841	3,024
Unrecognized losses for more than 12 months, Cost	3,948	874
Unrecognized losses for more than 12 months, Estimated Fair value	3,930	873
Unrecognized gains, Cost	5,957	12,175
Unrecognized gains, Estimated Fair value	6,001	12,267
Available-for-sale securities, Cost	15,758	16,106
Available-for-sale securities, Estimated Fair Value	15,772	16,164
Level 2 | Corporate notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months	9,155	1,993
Unrecognized losses for less than 12 months, Estimated Fair value	9,049	1,971
Unrecognized losses for more than 12 months, Cost	316	201
Unrecognized losses for more than 12 months, Estimated Fair value	315	200
Unrecognized gains, Cost	22,082	29,210
Unrecognized gains, Estimated Fair value	22,338	29,653
Available-for-sale securities, Cost	31,553	31,404
Available-for-sale securities, Estimated Fair Value	31,702	31,824
Level 2 | Municipal notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months	1,378	1,961
Unrecognized losses for less than 12 months, Estimated Fair value	1,360	1,948
Unrecognized losses for more than 12 months, Cost	1,045	620
Unrecognized losses for more than 12 months, Estimated Fair value	1,040	613
Unrecognized gains, Cost	5,689	6,702
Unrecognized gains, Estimated Fair value	5,745	6,794
Available-for-sale securities, Cost	8,112	9,283
Available-for-sale securities, Estimated Fair Value	$ 8,145	$ 9,355