Short-term Investments (Available-For-Sale Securities Reported At Fair Value) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Total, Amortized Cost	$ 1,140,143	[1]	$ 750,466	[2]
Total, Unrealized Gains	1,615	[1]	2,640	[2]
Total, Unrealized Losses	(1,568)	[1]	(157)	[2]
Total, Estimated Fair Value	1,140,190	[1]	752,949	[2]
Cash and Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, Estimated Fair Value	439,100		290,300
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, Estimated Fair Value	695,900		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	24,024		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	24,024		10,500
Level 2 Securities [Member] | Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	324,422		150,322
Available-for-sale Securities, Gross Unrealized Gains	975		1,605
Available-for-sale Securities, Gross Unrealized Losses	(788)		(32)
Available-for-sale Securities, Estimated Fair Value	324,609		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	163,610		166,643
Available-for-sale Securities, Gross Unrealized Gains	352		285
Available-for-sale Securities, Gross Unrealized Losses	(18)		0
Available-for-sale Securities, Estimated Fair Value	163,944		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	207,852		138,340
Available-for-sale Securities, Gross Unrealized Gains	288		750
Available-for-sale Securities, Gross Unrealized Losses	(762)		(125)
Available-for-sale Securities, Estimated Fair Value	207,378		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	420,235		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	$ 420,235		$ 284,661

[1] Includes $439.1 million that is included in cash and cash equivalents, $695.9 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.