Cash, Cash Equivalents and Investments (Details) (USD $)																													6 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2012 Cash Equivalents:	Jun. 30, 2012 Short-term Investments:	Dec. 31, 2011 Short-term Investments:	Jun. 30, 2012 Long-term Investments:	Dec. 31, 2011 Long-term Investments:	Jun. 30, 2012 Money market funds Cash Equivalents:	Dec. 31, 2011 Money market funds Cash Equivalents:	Jun. 30, 2012 Corporate bonds Short-term Investments:	Dec. 31, 2011 Corporate bonds Short-term Investments:	Jun. 30, 2012 Municipal bonds Short-term Investments:	Dec. 31, 2011 Municipal bonds Short-term Investments:	Jun. 30, 2012 Variable-rate demand notes Short-term Investments:	Dec. 31, 2011 Variable-rate demand notes Short-term Investments:	Jun. 30, 2012 U.S. government agency Short-term Investments:	Dec. 31, 2011 U.S. government agency Short-term Investments:	Jun. 30, 2012 U.S. Treasury bills Cash Equivalents:	Jun. 30, 2012 U.S. Treasury bills Short-term Investments:	Dec. 31, 2011 U.S. Treasury bills Short-term Investments:	Jun. 30, 2012 Asset-backed securities Short-term Investments:	Dec. 31, 2011 Asset-backed securities Short-term Investments:	Jun. 30, 2012 U.S. government bonds Short-term Investments:	Dec. 31, 2011 U.S. government bonds Short-term Investments:	Jun. 30, 2012 International government bonds Short-term Investments:	Dec. 31, 2011 International government bonds Short-term Investments:	Jun. 30, 2012 Auction rate securities Long-term Investments:	Dec. 31, 2011 Auction rate securities Long-term Investments:	Jun. 30, 2012 Auction rate securities Long-term Investments: Low end of range	Jun. 30, 2012 Auction rate securities Long-term Investments: High end of range	Dec. 31, 2011 Certificates of deposit Short-term Investments:
Cash, Cash Equivalents and Investments
Auction-rate securities guaranteed by the U.S. government	$ 10,500,000
Auction-rate securities privately insured	2,000,000
Auction-rate securities having credit rating 'AAA'	4,500,000
Auction-rate securities having credit rating 'AA'	6,000,000
Auction-rate securities having credit rating 'A'	2,000,000
Cash on hand, Cost	107,595,000	44,113,000
Cash on hand, Fair Value	107,595,000	44,113,000
Cash and cash equivalents, Cost	173,753,000	94,964,000
Cash and cash equivalents, Fair Value	173,753,000	94,964,000
Available-for-sale securities
Long-term investment interest (as a percent)																													0.26%	0.57%
Available-for-sale securities, Cost			66,158,000	137,930,000	212,574,000	12,525,000	19,225,000	49,917,000	50,851,000	62,993,000	75,189,000	32,495,000	56,915,000	8,980,000	41,280,000	900,000	19,820,000	16,241,000	7,149,000	8,600,000	10,655,000	5,743,000	12,647,000	2,507,000	2,111,000	950,000	12,525,000	19,225,000			1,570,000
Available-for-sale securities, Gross Unrealized Losses				(45,000)	(392,000)	(1,497,000)	(1,748,000)			(36,000)	(363,000)	(8,000)	(12,000)				(12,000)					(5,000)			(1,000)		(1,497,000)	(1,748,000)
Available-for-sale securities, Gross Unrealized Gains				262,000	344,000					197,000	234,000	45,000	81,000	1,000			28,000		1,000		10,000	1,000	8,000
Available-for-sale securities, Fair Value			$ 66,158,000	$ 138,147,000	$ 212,526,000	$ 11,028,000	$ 17,477,000	$ 49,917,000	$ 50,851,000	$ 63,154,000	$ 75,060,000	$ 32,532,000	$ 56,984,000	$ 8,981,000	$ 41,280,000	$ 900,000	$ 19,836,000	$ 16,241,000	$ 7,150,000	$ 8,600,000	$ 10,665,000	$ 5,739,000	$ 12,655,000	$ 2,507,000	$ 2,110,000	$ 950,000	$ 11,028,000	$ 17,477,000			$ 1,570,000