Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 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	$ 226,381	$ 359,097
Amortized Cost	226,449	359,112
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(68)	(15)
Cash, fair value	20,049	28,565
Cash, amortized cost	20,049	28,565
Cash, cash equivalents and marketable securities
Fair Value	246,430	387,662
Amortized Cost	246,498	387,677
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(68)	(15)
Money market funds
Cash equivalents and marketable securities
Fair Value	79,522	300,605
Amortized Cost	79,522	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	146,859	58,492
Amortized Cost	146,927	58,507
Gross Unrealized Gains	0	0
Gross Unrealized Losses	$ (68)	$ (15)
Weighted Rate of Return (as a percent)	0.17%	0.15%