Short-Term Investments (Available-For-Sale Securities Reported At Fair Value) (Details) (Fair Value, Measurements, Recurring [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Total, Amortized Cost	$ 1,204,938	[1]	$ 750,466	[2]
Total, Gross Unrealized Gains	1,761	[1]	2,640	[2]
Total, Gross Unrealized Losses	(1,092)	[1]	(157)	[2]
Total, Estimated Fair Value	1,205,607	[1]	752,949	[2]
Cash and Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, Estimated Fair Value	605,000		290,300
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, Estimated Fair Value	595,400		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 and Money market funds, Amortized Cost	126,208		10,500
Cash and Money market funds, Gross Unrealized Gains	0		0
Cash and Money market funds, Gross Unrealized Losses	0		0
Cash and Money market funds, Fair Value Disclosure	126,208		10,500
Level 2 Securities [Member] | Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	316,465		150,322
Available-for-sale Securities, Gross Unrealized Gains	1,245		1,605
Available-for-sale Securities, Gross Unrealized Losses	(654)		(32)
Available-for-sale Securities, Estimated Fair Value	317,056		151,895
Level 2 Securities [Member] | Government securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	143,812		166,643
Available-for-sale Securities, Gross Unrealized Gains	287		285
Available-for-sale Securities, Gross Unrealized Losses	(18)		0
Available-for-sale Securities, Estimated Fair Value	144,081		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	93,118		138,340
Available-for-sale Securities, Gross Unrealized Gains	229		750
Available-for-sale Securities, Gross Unrealized Losses	(418)		(125)
Available-for-sale Securities, Estimated Fair Value	92,929		138,965
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 and Money market funds, Amortized Cost	483,959		284,661
Cash and Money market funds, Gross Unrealized Gains	0		0
Cash and Money market funds, Gross Unrealized Losses	0		0
Cash and Money market funds, Fair Value Disclosure	$ 483,959		$ 284,661

[1] 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.

[2] Includes $290.3 million 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.