Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Summary of available-for-sale securities
Available-for-sale securities, Cost	$ 67,090	$ 68,164
Available-for-sale securities, Estimated Fair Value	67,836	68,566
Level 1 [Member] | Certificates of deposit [Member]
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	711	3,646
Unrecognized losses for less than 12 months, Estimated Fair value	707	3,605
Unrecognized gains, Cost	5,963	3,525
Unrecognized gains, Estimated Fair value	6,017	3,551
Available-for-sale securities, Cost	6,674	7,171
Available-for-sale securities, Estimated Fair Value	6,724	7,156
Level 2 [Member] | U.S. Treasury and agency notes [Member]
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	3,925	4,352
Unrecognized losses for less than 12 months, Estimated Fair value	3,888	4,338
Unrecognized losses for more than 12 months, Cost	0	1,002
Unrecognized losses for more than 12 months, Estimated Fair value	0	1,000
Unrecognized gains, Cost	15,220	16,479
Unrecognized gains, Estimated Fair value	15,348	16,660
Available-for-sale securities, Cost	19,145	21,833
Available-for-sale securities, Estimated Fair Value	19,236	21,998
Level 2 [Member] | Corporate notes [Member]
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	1,077	9,230
Unrecognized losses for less than 12 months, Estimated Fair value	1,069	9,098
Unrecognized losses for more than 12 months, Cost	0	1,539
Unrecognized losses for more than 12 months, Estimated Fair value	0	1,494
Unrecognized gains, Cost	30,334	19,369
Unrecognized gains, Estimated Fair value	30,858	19,738
Available-for-sale securities, Cost	31,411	30,138
Available-for-sale securities, Estimated Fair Value	31,927	30,330
Level 2 [Member] | Municipal notes [Member]
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	1,291	1,177
Unrecognized losses for less than 12 months, Estimated Fair value	1,277	1,165
Unrecognized losses for more than 12 months, Cost	865	881
Unrecognized losses for more than 12 months, Estimated Fair value	861	873
Unrecognized gains, Cost	7,704	6,964
Unrecognized gains, Estimated Fair value	7,811	7,044
Available-for-sale securities, Cost	9,860	9,022
Available-for-sale securities, Estimated Fair Value	$ 9,949	$ 9,082