Short-term Investments - Available-For-Sale Securities Reported At Fair Value (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014		Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Total, Amortized Cost	$ 1,718,508	[1]	$ 1,204,938	[2]
Total, Unrealized Gains	1,578	[1]	1,761	[2]
Total, Unrealized Losses	(18)	[1]	(1,092)	[2]
Total, Estimated Fair Value	1,720,068	[1]	1,205,607	[2]
Cash and Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, Estimated Fair Value	1,214,200		605,000
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, Estimated Fair Value	500,100		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	106,094		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	106,094		126,208
Level 2 Securities [Member] | Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	309,328		316,465
Available-for-sale Securities, Gross Unrealized Gains	1,266		1,245
Available-for-sale Securities, Gross Unrealized Losses	(17)		(654)
Available-for-sale Securities, Estimated Fair Value	310,577		317,056
Level 2 Securities [Member] | Government securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	163,871		143,812
Available-for-sale Securities, Gross Unrealized Gains	311		287
Available-for-sale Securities, Gross Unrealized Losses	(1)		(18)
Available-for-sale Securities, Estimated Fair Value	164,181		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	1,534		93,118
Available-for-sale Securities, Gross Unrealized Gains	1		229
Available-for-sale Securities, Gross Unrealized Losses	0		(418)
Available-for-sale Securities, Estimated Fair Value	1,535		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	23,828
Available-for-sale Securities, Gross Unrealized Gains	0
Available-for-sale Securities, Gross Unrealized Losses	0
Available-for-sale Securities, Estimated Fair Value	23,828
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			17,951
Available-for-sale Securities, Gross Unrealized Gains			0
Available-for-sale Securities, Gross Unrealized Losses			(2)
Available-for-sale Securities, Estimated Fair Value			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	1,113,853		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	$ 1,113,853		$ 483,959

[1] Includes $1,214.2 million that is included in cash and cash equivalents, $500.1 million included in short-term investments and $5.7 million of restricted cash that is included in other non-current assets related 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.