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Fair Value Measurements - Assets measured on a recurring basis - (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2015
USD ($)
Carrying Value  
Assets, Fair Value Disclosure [Abstract]  
Total assets measured at fair value $ 222,637
Level 1 | Estimate of Fair Value  
Assets, Fair Value Disclosure [Abstract]  
Total assets measured at fair value 187,582
Level 2 | Estimate of Fair Value  
Assets, Fair Value Disclosure [Abstract]  
Total assets measured at fair value 35,055
Level 3  
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]  
Balance, begining 6,305
Changes in fair value 9,750
Reclassification to Series A Preferred Stock (16,055)
U.S. Treasury notes | Carrying Value  
Assets, Fair Value Disclosure [Abstract]  
Investments, Fair Value Disclosure 157,981
U.S. Treasury notes | Level 1 | Estimate of Fair Value  
Assets, Fair Value Disclosure [Abstract]  
Investments, Fair Value Disclosure 157,981
U. S. Government agency bonds | Carrying Value  
Assets, Fair Value Disclosure [Abstract]  
Investments, Fair Value Disclosure 35,055
U. S. Government agency bonds | Level 2 | Estimate of Fair Value  
Assets, Fair Value Disclosure [Abstract]  
Investments, Fair Value Disclosure 35,055
Money market funds  
Assets, Fair Value Disclosure [Abstract]  
Cash and cash equivalents 29,601
Money market funds | Carrying Value  
Assets, Fair Value Disclosure [Abstract]  
Cash and cash equivalents 29,601
Money market funds | Level 1 | Estimate of Fair Value  
Assets, Fair Value Disclosure [Abstract]  
Cash and cash equivalents $ 29,601