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Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended |
--- | --- | ---
Dec. 31, 2013 | Dec. 31, 2012
Cash equivalents and marketable securities |  |
Fair Value | $ 359,097 | $ 183,915
Amortized Cost | 359,112 | 183,926
Gross Unrealized Gains | 0 | 4
Gross Unrealized Losses | (15) | (15)
Cash, fair value | 28,565 | 19,415
Cash | 28,565 | 19,415
Cash, cash equivalents and marketable securities |  |
Fair Value | 387,662 | 203,330
Amortized Cost | 387,677 | 203,341
Gross Unrealized Gains | 0 | 4
Gross Unrealized Loss | (15) | (15)
Money market funds |  |
Cash equivalents and marketable securities |  |
Fair Value | 300,605 | 126,570
Amortized Cost | 300,605 | 126,570
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 |  |
Maximum maturity period of available-for-sale securities (in years) | 3 years |
Maximum remaining maturity period of available-for-sale securities (in years) | 1 year |
Fair Value | 58,492 | 57,345
Amortized Cost | 58,507 | 57,356
Gross Unrealized Gains | 0 | 4
Gross Unrealized Losses | (15) | (15)
Weighted Rate of Return (as a percent) | 0.15% | 0.17%
Unrealized gain (loss), net |  |
Less than one year, Fair value | 53,491 | 51,819
Less than one year, Gross unrealized loss | (15) | (15)
Cash equivalents |  |
Cash equivalents and marketable securities |  |
Maximum remaining maturity period of available-for-sale securities (in years) | 1 year |
Fair Value | 310,131 | 129,569
Short term marketable securities |  |
Cash equivalents and marketable securities |  |
Fair Value | $ 48,966 | $ 54,346

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