Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Cash equivalents and marketable securities
Maximum maturity period of available-for-sale securities (in years)	3 years	3 years
Maximum remaining maturity period of available-for-sale securities (in years)	1 year	1 year
Fair Value	$ 376,116	$ 359,097
Amortized Cost	376,124	359,112
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(9)	(15)
Cash, fair value	27,308	28,565
Cash, amortized cost	27,308	28,565
Cash, cash equivalents and marketable securities
Fair Value	403,424	387,662
Amortized Cost	403,432	387,677
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(9)	(15)
Money market funds
Cash equivalents and marketable securities
Fair Value	317,769	300,605
Amortized Cost	317,769	300,605
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Weighted Rate of Return (as a percent)	0.01%	0.01%
Corporate notes, bonds and commercial paper
Cash equivalents and marketable securities
Fair Value	58,347	58,492
Amortized Cost	58,355	58,507
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(9)	(15)
Weighted Rate of Return (as a percent)	0.13%	0.15%
Cash equivalents and marketable securities, Continuous unrealized loss position
Less than one year, Fair Value	43,151	53,491
Unrealized gain (loss), Gross
Less than one year, Gross Unrealized Loss	(9)	(15)
Cash equivalents
Cash equivalents and marketable securities
Fair Value	319,268	310,131
Short term marketable securities
Cash equivalents and marketable securities
Fair Value	$ 56,848	$ 48,966