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Organization and Significant Accounting Policies, Fair Value Measurements on a Recurring Basis (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Organization and Significant Accounting Policies [Abstract]    
Transfers from Level 1 to Level 2 $ 0 $ 0
Transfers from Level 2 to Level 1 0 0
Fair Value Measurements [Abstract]    
Available-for-sale securities   592,211
Recurring Basis [Member]    
Fair Value Measurements [Abstract]    
Cash equivalents [1] 86,262 54,137
Investment in Regulus Therapeutics Inc.   2,414
Total 994,798 646,348
Recurring Basis [Member] | Corporate Debt Securities [Member]    
Fair Value Measurements [Abstract]    
Available-for-sale securities 647,461 [2] 396,221 [3]
Recurring Basis [Member] | Corporate Debt Securities [Member] | Cash and Cash Equivalents [Member]    
Fair Value Measurements [Abstract]    
Available-for-sale securities 11,900  
Recurring Basis [Member] | Debt Securities issued by U.S. Government Agencies [Member]    
Fair Value Measurements [Abstract]    
Available-for-sale securities [3] 136,325 55,179
Recurring Basis [Member] | Debt Securities issued by the U.S. Treasury [Member]    
Fair Value Measurements [Abstract]    
Available-for-sale securities [3] 30,818 29,286
Recurring Basis [Member] | Debt Securities issued by the U.S. Treasury [Member] | Cash and Cash Equivalents [Member]    
Fair Value Measurements [Abstract]    
Available-for-sale securities 3,500 9,300
Recurring Basis [Member] | Debt Securities issued by States of the U.S. and Political Subdivisions of the States [Member]    
Fair Value Measurements [Abstract]    
Available-for-sale securities 93,932 [4] 109,111 [5]
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member]    
Fair Value Measurements [Abstract]    
Cash equivalents 86,262 54,137
Investment in Regulus Therapeutics Inc.   2,414
Total 117,080 85,837
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member] | Corporate Debt Securities [Member]    
Fair Value Measurements [Abstract]    
Available-for-sale securities 0 0
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member] | Debt Securities issued by U.S. Government Agencies [Member]    
Fair Value Measurements [Abstract]    
Available-for-sale securities 0 0
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member] | Debt Securities issued by the U.S. Treasury [Member]    
Fair Value Measurements [Abstract]    
Available-for-sale securities 30,818 29,286
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member] | Debt Securities issued by States of the U.S. and Political Subdivisions of the States [Member]    
Fair Value Measurements [Abstract]    
Available-for-sale securities 0 0
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member]    
Fair Value Measurements [Abstract]    
Cash equivalents 0 0
Investment in Regulus Therapeutics Inc.   0
Total 877,718 560,511
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Corporate Debt Securities [Member]    
Fair Value Measurements [Abstract]    
Available-for-sale securities 647,461 396,221
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Debt Securities issued by U.S. Government Agencies [Member]    
Fair Value Measurements [Abstract]    
Available-for-sale securities 136,325 55,179
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Debt Securities issued by the U.S. Treasury [Member]    
Fair Value Measurements [Abstract]    
Available-for-sale securities 0 0
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Debt Securities issued by States of the U.S. and Political Subdivisions of the States [Member]    
Fair Value Measurements [Abstract]    
Available-for-sale securities 93,932 109,111
Recurring Basis [Member] | Significant Unobservable Inputs (Level 3) [Member]    
Fair Value Measurements [Abstract]    
Total $ 0 $ 0
[1] Included in cash and cash equivalents on our consolidated balance sheet.
[2] $11.9 million included in cash and cash equivalents on our consolidated balance sheet, with the difference included in short-term investments on our consolidated balance sheet.
[3] Included in short-term investments on our consolidated balance sheet.
[4] $3.5 million included in cash and cash equivalents on our consolidated balance sheet, with the difference included in short-term investments on our consolidated balance sheet.
[5] $9.3 million included in cash and cash equivalents on our consolidated balance sheet, with the difference included in short-term investments on our consolidated balance sheet.