Short-term Investments (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost	$ 58,819	$ 130,971
Available-for-sale securities, gross unrealized gains	2	21
Available-for-sale securities, gross unrealized losses	(263)	(975)
Available-for-sale securities, fair value	58,558us-gaap_AvailableForSaleSecurities	130,017us-gaap_AvailableForSaleSecurities
Available-for-sale Securities, Gross Realized Gains		1,174	31
Available-for-sale Securities, Gross Realized Losses	2,188	981	555
Proceeds from Sale and Maturity of Marketable Securities	$ 182,115	$ 239,370	$ 319,811
Minimum [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment Maturity Date		Dec. 31, 2014
Maximum [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment Maturity Date	Dec. 31, 2015	Dec. 31, 2015

The maturity date of the investment.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

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