Investments (Schedule of Available-for-sale Securities) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 279,083	$ 251,847
Gross unrealized gains	396	320
Gross unrealized losses	(149)	(290)
Fair market value	279,330us-gaap_AvailableForSaleSecurities	251,877us-gaap_AvailableForSaleSecurities
Federal agency notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	168,485	143,132
Gross unrealized gains	97	40
Gross unrealized losses	0	(33)
Fair market value	168,582	143,139
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	110,130	108,371
Gross unrealized gains	2	1
Gross unrealized losses	(149)	(257)
Fair market value	109,983	108,115
Mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	468	344
Gross unrealized gains	297	279
Gross unrealized losses	0	0
Fair market value	$ 765	$ 623

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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

-Publisher FASB

-Name Accounting Standards Codification

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

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This item represents the cost of debt and equity 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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