Cash, Cash Equivalents, Short-Term Investments And Fair Value Measurment (Available-For-Sale Securities Reported At Fair Value) (Details) (USD $)	Jun. 30, 2012		Mar. 31, 2012	Dec. 31, 2011		Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 816,779,000			$ 801,218,000
Gross Unrealized Gains	1,508,000			1,359,000
Gross Unrealized Losses	(75,000)			(270,000)
Estimated Fair Value	818,212,000			802,307,000
Less: Restricted cash	(4,869,000)	[1]		(4,496,000)	[2]
Total cash, cash equivalents and short-term investments	813,343,000			797,811,000
Cash and cash equivalents	402,251,000		395,992,000	508,053,000		175,207,000	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	402,192,000			388,941,000
Gross Unrealized Gains	0			0
Gross Unrealized Losses	0			0
Estimated Fair Value	402,192,000			388,941,000
Cash And Cash Equivalent [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents				119,100,000
Level 1 Securities [Member] | Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,928,000	[1]		123,608,000	[2]
Gross Unrealized Gains	0	[1]		0	[2]
Gross Unrealized Losses	0	[1]		0	[2]
Estimated Fair Value	4,928,000	[1]		123,608,000	[2]
Level 2 Securities [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	108,956,000	[3]		112,264,000	[3]
Gross Unrealized Gains	807,000	[3]		603,000	[3]
Gross Unrealized Losses	(20,000)	[3]		(214,000)	[3]
Estimated Fair Value	109,743,000	[3]		112,653,000	[3]
Level 2 Securities [Member] | US Treasury and Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	210,477,000	[3]		175,464,000	[3]
Gross Unrealized Gains	281,000	[3]		694,000	[3]
Gross Unrealized Losses	(38,000)	[3]		(56,000)	[3]
Estimated Fair Value	210,720,000	[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	90,226,000	[3]		941,000	[3]
Gross Unrealized Gains	420,000	[3]		62,000	[3]
Gross Unrealized Losses	(17,000)	[3]		0	[3]
Estimated Fair Value	90,629,000	[3]		1,003,000	[3]
Workers Compensation Deposits [Member]
Schedule of Available-for-sale Securities [Line Items]
Long-term restricted cash				$ 4,500,000

[1] Includes $4.9 million of restricted cash classified in other non-current assets.

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

[3] Included in short-term investments.