Fair Value Measurements - Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,171,181	$ 912,149
Gross Unrealized Gains	1,101	1,248
Gross Unrealized Losses	(1,607)	(366)
Aggregate Fair Value	1,170,675	913,031
Short-Term Marketable Securities	449,404	340,005
Long-Term Marketable Securities	721,271	573,026
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	225	222
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Aggregate Fair Value	225	222
Short-Term Marketable Securities	172	173
Long-Term Marketable Securities	53	49
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	24,986
Gross Unrealized Gains	0
Gross Unrealized Losses	(12)
Aggregate Fair Value	24,974
Short-Term Marketable Securities	24,974
Long-Term Marketable Securities	0
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	874,265	736,945
Gross Unrealized Gains	1,087	1,197
Gross Unrealized Losses	(1,161)	(281)
Aggregate Fair Value	874,191	737,861
Short-Term Marketable Securities	358,887	278,318
Long-Term Marketable Securities	515,304	459,543
U.S. government agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	271,705	174,982
Gross Unrealized Gains	14	51
Gross Unrealized Losses	(434)	(85)
Aggregate Fair Value	271,285	174,948
Short-Term Marketable Securities	65,371	61,514
Long-Term Marketable Securities	$ 205,914	$ 113,434

- Definition

Investments in debt and equity securities which are categorized neither as held-to-maturity nor trading and which are intended to be sold or mature more than one year from the balance sheet date or operating cycle, if longer. Such securities are reported at fair value; unrealized gains (losses) related to Available-for-sale Securities are excluded from earnings and reported in a separate component of shareholders' equity (other comprehensive income), unless the Available-for-sale security is designated as a hedge or is determined to have had an other than temporary decline in fair value below its amortized cost basis. All or a portion of the unrealized holding gain (loss) of an Available-for-sale security that is designated as being hedged in a fair value hedge is recognized in earnings during the period of the hedge, as are other than temporary declines in fair value below the cost basis for investments in equity securities and debt securities that an entity intends to sell or it is more likely than not that it will be required to sell before the recovery of its amortized cost basis. Other than temporary declines in fair value below the cost basis for debt securities categorized as Available-for-sale that an entity does not intend to sell and for which it is not more likely than not that the entity will be required to sell before the recovery of its amortized cost basis are bifurcated into credit losses and losses related to all other factors. Other than temporary declines in fair value below cost basis related to credit losses are recognized in earnings, and losses related to all other factors are recognized in other comprehensive income.

+ References+ Details