Short-term Investments (Available-For-Sale Securities Reported At Fair Value) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Total, Amortized Cost	$ 1,086,606	[1]	$ 750,466	[2]
Total, Unrealized Gains	1,374	[1]	2,640	[2]
Total, Unrealized Losses	(2,667)	[1]	(157)	[2]
Total, Estimated Fair Value	1,085,313	[1]	752,949	[2]
Cash and Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, Estimated Fair Value	370,700		290,300
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, Estimated Fair Value	709,400		457,800
Non-current Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, Estimated Fair Value	5,200		4,800
Level 1 Securities [Member] | Money market funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash, Money market and Government and agency securities classified as cash equivalents, Amortized Cost	7,354		10,500
Cash, Money market and Government and agency securities classified as cash equivalents, Gross Unrealized Gains	0		0
Cash, Money market and Government and agency securities classified as cash equivalents, Gross Unrealized Losses	0		0
Cash, Money market and Government and agency securities classified as cash equivalents, Estimated Fair Value	7,354		10,500
Level 2 Securities [Member] | Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	332,174		150,322
Available-for-sale Securities, Gross Unrealized Gains	918		1,605
Available-for-sale Securities, Gross Unrealized Losses	(1,714)		(32)
Available-for-sale Securities, Estimated Fair Value	331,378		151,895
Level 2 Securities [Member] | Government and agency securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	181,638		166,643
Available-for-sale Securities, Gross Unrealized Gains	180		285
Available-for-sale Securities, Gross Unrealized Losses	(156)		0
Available-for-sale Securities, Estimated Fair Value	181,662		166,928
Level 2 Securities [Member] | Asset and mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	196,913		138,340
Available-for-sale Securities, Gross Unrealized Gains	276		750
Available-for-sale Securities, Gross Unrealized Losses	(797)		(125)
Available-for-sale Securities, Estimated Fair Value	196,392		138,965
Cash [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash, Money market and Government and agency securities classified as cash equivalents, Amortized Cost	368,527		284,661
Cash, Money market and Government and agency securities classified as cash equivalents, Gross Unrealized Gains	0		0
Cash, Money market and Government and agency securities classified as cash equivalents, Gross Unrealized Losses	0		0
Cash, Money market and Government and agency securities classified as cash equivalents, Estimated Fair Value	$ 368,527		$ 284,661

[1] Includes $370.7 million that is included in cash and cash equivalents, $709.4 million included in short-term investments and $5.2 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.