Short-Term Investments And Fair Value Measurement (Available-For-Sale Securities Reported At Fair Value) (Details) (USD $)	Mar. 31, 2012		Dec. 31, 2011		Mar. 31, 2011	Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 808,207,000		$ 801,218,000
Gross Unrealized Gains	1,414,000		1,359,000
Gross Unrealized Losses	(227,000)		(270,000)
Estimated Fair Value	809,394,000		802,307,000
Less: Long-term restricted cash	(4,868,000)	[1]	(4,496,000)	[2]
Total cash, cash equivalents and short-term investments	804,526,000		797,811,000
Cash and cash equivalents	395,992,000		508,053,000		150,419,000	194,499,000
Long-term restricted cash	4,900,000
Cash [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	394,726,000		388,941,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Estimated Fair Value	394,726,000		388,941,000
Cash And Cash Equivalent [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	1,200,000		119,100,000
Level 1 Securities [Member] | Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,134,000	[1]	123,608,000	[2]
Gross Unrealized Gains	0	[1]	0	[2]
Gross Unrealized Losses	0	[1]	0	[2]
Estimated Fair Value	6,134,000	[1]	123,608,000	[2]
Level 2 Securities [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	104,998,000	[3]	112,264,000	[3]
Gross Unrealized Gains	769,000	[3]	603,000	[3]
Gross Unrealized Losses	(65,000)	[3]	(214,000)	[3]
Estimated Fair Value	105,702,000	[3]	112,653,000	[3]
Level 2 Securities [Member] | US Treasury and Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	253,897,000	[3]	175,464,000	[3]
Gross Unrealized Gains	417,000	[3]	694,000	[3]
Gross Unrealized Losses	(147,000)	[3]	(56,000)	[3]
Estimated Fair Value	254,167,000	[3]	176,102,000	[3]
Level 2 Securities [Member] | Asset And Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	48,452,000	[3]	941,000	[3]
Gross Unrealized Gains	228,000	[3]	62,000	[3]
Gross Unrealized Losses	(15,000)	[3]	0	[3]
Estimated Fair Value	48,665,000	[3]	1,003,000	[3]
Workers Compensation Deposits [Member]
Schedule of Available-for-sale Securities [Line Items]
Long-term restricted cash			4,500,000
Restricted cash other current assets			$ 0

[1] Includes $1.2 million classified in cash and cash equivalents and $4.9 million classified in other non-current assets.

[2] Includes $119.1 million classified in cash and cash equivalents and $4.5 million classified in other current assets and non-current assets.

[3] Included in short-term investments.