Short-term Investments (Available-For-Sale Securities Reported At Fair Value) (Details) (USD $)	Mar. 31, 2013		Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,029,555,000	[1]	$ 750,466,000	[2]
Gross Unrealized Gains	2,408,000	[1]	2,640,000	[2]
Gross Unrealized Losses	(489,000)	[1]	(157,000)	[2]
Estimated Fair Value	1,031,474,000	[1]	752,949,000	[2]
Cash [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	310,813,000		284,661,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Estimated Fair Value	310,813,000		284,661,000
Level 1 Securities [Member] | Money market funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,017,000		10,500,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Estimated Fair Value	7,017,000		10,500,000
Level 2 Securities [Member] | Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	318,573,000		150,322,000
Gross Unrealized Gains	1,760,000		1,605,000
Gross Unrealized Losses	(291,000)		(32,000)
Estimated Fair Value	320,042,000		151,895,000
Level 2 Securities [Member] | Government and agency securities classified as cash equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	105,823,000
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Estimated Fair Value	105,823,000
Level 2 Securities [Member] | Government and agency securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	114,110,000		166,643,000
Gross Unrealized Gains	179,000		285,000
Gross Unrealized Losses	0		0
Estimated Fair Value	114,289,000		166,928,000
Level 2 Securities [Member] | Asset and mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	173,219,000		138,340,000
Gross Unrealized Gains	469,000		750,000
Gross Unrealized Losses	(198,000)		(125,000)
Estimated Fair Value	173,490,000		138,965,000
Cash and Cash Equivalents [Member] | Cash And Cash Equivalent [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	418,100,000		290,300,000
Short-term Investments [Member] | Cash And Cash Equivalent [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	607,800,000		457,800,000
Restricted Cash [Member]
Schedule of Available-for-sale Securities [Line Items]
Restricted cash	$ 5,600,000		$ 4,800,000

[1] Includes $418.1 million that is included in cash and cash equivalents, $607.8 million included in short-term investments and $5.6 million of restricted cash that is included in other non-current assets related to workers compensation deposits.

[2] 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.