Short-term Investments - Available-For-Sale Securities Reported At Fair Value (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Total, Amortized Cost	$ 1,671,783	[1]	$ 1,204,938	[2]
Total, Unrealized Gains	895	[1]	1,761	[2]
Total, Unrealized Losses	(152)	[1]	(1,092)	[2]
Total, Estimated Fair Value	1,672,526	[1]	1,205,607	[2]
Cash and Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, Estimated Fair Value	1,183,200		605,000
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, Estimated Fair Value	483,600		595,400
Non-current Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, Estimated Fair Value	5,700		5,200
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	122,936		126,208
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	122,936		126,208
Level 2 Securities [Member] | Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	292,410		316,465
Available-for-sale Securities, Gross Unrealized Gains	718		1,245
Available-for-sale Securities, Gross Unrealized Losses	(99)		(654)
Available-for-sale Securities, Estimated Fair Value	293,029		317,056
Level 2 Securities [Member] | Government securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	166,991		143,812
Available-for-sale Securities, Gross Unrealized Gains	169		287
Available-for-sale Securities, Gross Unrealized Losses	(53)		(18)
Available-for-sale Securities, Estimated Fair Value	167,107		144,081
Level 2 Securities [Member] | Asset and mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	370		93,118
Available-for-sale Securities, Gross Unrealized Gains	0		229
Available-for-sale Securities, Gross Unrealized Losses	0		(418)
Available-for-sale Securities, Estimated Fair Value	370		92,929
Level 2 Securities [Member] | Certificates of deposits and commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	6,605
Available-for-sale Securities, Gross Unrealized Gains	0
Available-for-sale Securities, Gross Unrealized Losses	0
Available-for-sale Securities, Estimated Fair Value	6,605
Level 2 Securities [Member] | Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost			23,425
Available-for-sale Securities, Gross Unrealized Gains			0
Available-for-sale Securities, Gross Unrealized Losses			0
Available-for-sale Securities, Estimated Fair Value			23,425
Level 2 Securities [Member] | Agency securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	16,483		17,951
Available-for-sale Securities, Gross Unrealized Gains	8		0
Available-for-sale Securities, Gross Unrealized Losses	0		(2)
Available-for-sale Securities, Estimated Fair Value	16,491		17,949
Cash [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash, Money market and Government and agency securities classified as cash equivalents, Amortized Cost	1,065,988		483,959
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	$ 1,065,988		$ 483,959

[1] Includes $1,183.2 million that is included in cash and cash equivalents, $483.6 million included in short-term investments and $5.7 million of restricted cash that is included in other non-current assets related primarily to workers compensation deposits.

[2] Includes $605.0 million that is included in cash and cash equivalents, $595.4 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.