Marketable Securities (Details 2) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Fair value of financial instruments
Gain on sale of mortgage-backed secruities	$ 1,800
Long term investment (Note 5)	35,248
Marketable securities realized gains (losses)
Net realized gains from sale of restricted cash and investment and marketable securities	1,800	$ 0	$ 0
Recurring \| Level 1
Fair value of financial instruments
Cash and cash equivalents	521,439	452,297
Long term investment (Note 5)	35,248
Total assets	556,687	452,297
Recurring \| Level 2
Fair value of financial instruments
Corporate debt securities	186,344	144,402
Mortgage-backed securities		3,564
Total assets	186,344	147,966
Fair Value \| Recurring
Fair value of financial instruments
Cash and cash equivalents	521,439	452,297
Corporate debt securities	186,344	144,402
Long term investment (Note 5)	35,248
Mortgage-backed securities		3,564
Total assets	$ 743,031	$ 600,263

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Net gain (loss) resulting from the sales of securitized, pay-through debt securities collateralized by real estate mortgage loans (mortgages).

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Fair value of securitized, pay-through debt securities collateralized by real estate mortgage loans classified as available-for-sale.

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