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Cash and Investments (Narrative) (Details)
12 Months Ended
Dec. 31, 2016
USD ($)
account
investment
affiliate
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Investment [Line Items]      
Number of required investment accounts | account 1    
Number of affiliate banks | affiliate 1    
Gain (loss) on sale of available-for-sale securities $ 0 $ 0 $ 0
Other-than-temporary impairment charges on available-for-sale securities $ 0 0 0
Number of investments in unrealized loss position | investment 86    
Gross unrealized losses $ (322,000) (130,000)  
Unrealized loss position, aggregate fair value 172,100,000    
Realized gain on sale of cost method investment   0 $ 0
Carrying value of cost method investments 0 0  
Akarna Therapeutics, Ltd. [Member]      
Investment [Line Items]      
Realized gain on sale of cost method investment $ 2,500,000    
Cost method investment ownership percentage 9.00%    
Corporate Bonds [Member]      
Investment [Line Items]      
Gross unrealized losses $ (308,000) (118,000)  
Unrealized loss position, aggregate fair value 143,500,000    
Silicon Valley Bank Term Loan and Line of Credit [Member]      
Investment [Line Items]      
Collateral balance $ 81,600,000 $ 81,600,000