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Investments, Summary of Investments (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Summary of Investments [Abstract]    
Cost   $ 593,872
Gross unrealized gains   348
Gross unrealized losses   (2,009)
Estimated fair value   592,211
Securities with Maturity of One Year or Less [Member]    
Summary of Investments [Abstract]    
Cost [1] $ 643,193 323,708
Gross unrealized gains 6 29
Gross unrealized losses (1,103) (319)
Estimated fair value 642,096 323,418
Securities with Maturity of More than One Year [Member]    
Summary of Investments [Abstract]    
Cost [1] 268,401 268,031
Gross unrealized gains 8 38
Gross unrealized losses (1,969) (1,690)
Estimated fair value 266,440 266,379
Available-for-sale Securities [Member]    
Summary of Investments [Abstract]    
Cost [1] 911,594 591,739
Gross unrealized gains 14 67
Gross unrealized losses (3,072) (2,009)
Estimated fair value 908,536 589,797
Corporate Debt Securities [Member] | Securities with Maturity of One Year or Less [Member]    
Summary of Investments [Abstract]    
Cost [1] 500,599 [2] 195,087
Gross unrealized gains 2 25
Gross unrealized losses (752) (161)
Estimated fair value 499,849 194,951
Corporate Debt Securities [Member] | Securities with Maturity of More than One Year [Member]    
Summary of Investments [Abstract]    
Cost [1] 148,663 202,408
Gross unrealized gains 8 36
Gross unrealized losses (1,059) (1,174)
Estimated fair value 147,612 201,270
Debt Securities issued by U.S. Government Agencies [Member] | Securities with Maturity of One Year or Less [Member]    
Summary of Investments [Abstract]    
Cost [1] 83,926 26,548
Gross unrealized gains 0 0
Gross unrealized losses (212) (10)
Estimated fair value 83,714 26,538
Debt Securities issued by U.S. Government Agencies [Member] | Securities with Maturity of More than One Year [Member]    
Summary of Investments [Abstract]    
Cost [1] 52,779 28,807
Gross unrealized gains 0 1
Gross unrealized losses (168) (167)
Estimated fair value 52,611 28,641
Debt Securities issued by the U.S. Treasury [Member] | Securities with Maturity of One Year or Less [Member]    
Summary of Investments [Abstract]    
Cost [1] 29,428 29,298
Gross unrealized gains 0 2
Gross unrealized losses (17) (14)
Estimated fair value 29,411 29,286
Debt Securities issued by the U.S. Treasury [Member] | Securities with Maturity of More than One Year [Member]    
Summary of Investments [Abstract]    
Cost [1] 1,409  
Gross unrealized gains 0  
Gross unrealized losses (2)  
Estimated fair value 1,407  
Debt Securities issued by States of the U.S. and Political Subdivisions of the States [Member] | Securities with Maturity of One Year or Less [Member]    
Summary of Investments [Abstract]    
Cost [1],[2] 29,240 72,775
Gross unrealized gains 4 2
Gross unrealized losses (122) (134)
Estimated fair value 29,122 72,643
Debt Securities issued by States of the U.S. and Political Subdivisions of the States [Member] | Securities with Maturity of More than One Year [Member]    
Summary of Investments [Abstract]    
Cost [1] 65,550 36,816
Gross unrealized gains 0 1
Gross unrealized losses (740) (349)
Estimated fair value $ 64,810 36,468
Equity Securities [Member]    
Summary of Investments [Abstract]    
Cost   2,133
Gross unrealized gains   281
Gross unrealized losses   0
Estimated fair value   2,414
Regulus Therapeutics Inc. [Member]    
Summary of Investments [Abstract]    
Cost   2,133
Gross unrealized gains   281
Gross unrealized losses   0
Estimated fair value   $ 2,414
[1] Our available-for-sale securities are held at amortized cost.
[2] Includes investments classified as cash equivalents on our consolidated balance sheet.