Balance Sheet Components - Unrealized Gains and Losses and Fair Value of Investments (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2015 security	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Marketable securities in loss position greater than 12 months	0
Maximum maturity of marketable securities		2 years
Weighted average remaining duration of marketable securities	6 months 26 days
U.S. government notes
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	209,232	209,671
Unrealized Gains	0	0
Unrealized Losses	(79)	(245)
Fair Value	209,153	209,426

Available for Sale Securities, Maximum Maturity

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Available for Sale Securities, Weighted Average Remaining Duration

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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Number of investment positions in available-for-sale investments that have been in a continuous unrealized loss position for greater than or equal to a year for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

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