Short-Term Investments (Available-For-Sale Securities Reported At Fair Value) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 750,466	[1]	$ 801,218	[2]
Gross Unrealized Gains	2,640	[1]	1,359	[2]
Gross Unrealized Losses	(157)	[1]	(270)	[2]
Estimated Fair Value	752,949	[1]	802,307	[2]
Cash [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	284,661		388,941
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Estimated Fair Value	284,661		388,941
Level 1 Securities [Member] | Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,500		123,608
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Estimated Fair Value	10,500		123,608
Level 2 Securities [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	150,322		112,264
Gross Unrealized Gains	1,605		603
Gross Unrealized Losses	(32)		(214)
Estimated Fair Value	151,895		112,653
Level 2 Securities [Member] | Government And Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	166,643		175,464
Gross Unrealized Gains	285		694
Gross Unrealized Losses	0		(56)
Estimated Fair Value	166,928		176,102
Level 2 Securities [Member] | Asset And Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	138,340		941
Gross Unrealized Gains	750		62
Gross Unrealized Losses	(125)		0
Estimated Fair Value	138,965		1,003
Cash and Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	290,300		508,100
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	457,800		289,800
Restricted Cash [Member] | Workers Compensation Deposits [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value			4,500
Other Non-current Assets [Member] | Workers Compensation Deposits [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	$ 4,800

[1] Includes $290.3 million that is included in cash and cash equivalents, $457.8 million included in short-term investments and $4.8 million of restricted cash that is included in other non-current assets related to workers compensation deposits.

[2] ncludes $508.1 million included in cash and cash equivalents, $289.8 million included in short-term investments and $4.5 million of restricted cash that is included in other current assets and other non-current assets.