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Investments, Summary of Investments (Details) - USD ($)
$ in Thousands
6 Months Ended 12 Months Ended
Jun. 30, 2020
Dec. 31, 2019
Summary of Investments [Abstract]    
Cost $ 1,920,447 $ 1,846,074
Gross unrealized gains 25,562 5,007
Gross unrealized losses (2,514) (1,166)
Estimated fair value 1,943,495 1,849,915
Available-for-sale Securities [Member]    
Summary of Investments [Abstract]    
Cost [1] 1,900,716 1,831,362
Gross unrealized gains 16,244 5,007
Gross unrealized losses (192) (296)
Estimated fair value 1,916,768 1,836,073
Available-for-sale Securities [Member] | Securities with Maturity of One Year or Less [Member]    
Summary of Investments [Abstract]    
Cost [1] 1,187,843 1,206,616
Gross unrealized gains 5,984 2,012
Gross unrealized losses (36) (118)
Estimated fair value 1,193,791 1,208,510
Available-for-sale Securities [Member] | Securities with Maturity of More than One Year [Member]    
Summary of Investments [Abstract]    
Cost [1] 712,873 624,746
Gross unrealized gains 10,260 2,995
Gross unrealized losses (156) (178)
Estimated fair value 722,977 627,563
Corporate Debt Securities [Member] | Securities with Maturity of One Year or Less [Member]    
Summary of Investments [Abstract]    
Cost [1],[2] 652,555 669,665
Gross unrealized gains 4,275 1,451
Gross unrealized losses (17) (43)
Estimated fair value 656,813 671,073
Corporate Debt Securities [Member] | Securities with Maturity of More than One Year [Member]    
Summary of Investments [Abstract]    
Cost [1] 485,487 428,627
Gross unrealized gains 8,780 2,911
Gross unrealized losses (121) (43)
Estimated fair value 494,146 431,495
Debt Securities issued by U.S. Government Agencies [Member] | Securities with Maturity of One Year or Less [Member]    
Summary of Investments [Abstract]    
Cost [1] 139,810 188,216 [2]
Gross unrealized gains 673 303
Gross unrealized losses (9) (43)
Estimated fair value 140,474 188,476
Debt Securities issued by U.S. Government Agencies [Member] | Securities with Maturity of More than One Year [Member]    
Summary of Investments [Abstract]    
Cost [1] 118,538 140,988
Gross unrealized gains 521 57
Gross unrealized losses (28) (117)
Estimated fair value 119,031 140,928
Debt Securities issued by the U.S. Treasury [Member] | Securities with Maturity of One Year or Less [Member]    
Summary of Investments [Abstract]    
Cost [1],[2] 325,925 327,670
Gross unrealized gains 815 232
Gross unrealized losses (7) (27)
Estimated fair value 326,733 327,875
Debt Securities issued by the U.S. Treasury [Member] | Securities with Maturity of More than One Year [Member]    
Summary of Investments [Abstract]    
Cost [1] 58,334 35,822
Gross unrealized gains 588 9
Gross unrealized losses (4) (12)
Estimated fair value 58,918 35,819
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] 68,651 21,065 [2]
Gross unrealized gains 214 26
Gross unrealized losses (3) (5)
Estimated fair value 68,862 21,086
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] 50,514 19,309
Gross unrealized gains 371 18
Gross unrealized losses (3) (6)
Estimated fair value 50,882 19,321
Other Municipal Debt Securities [Member] | Securities with Maturity of One Year or Less [Member]    
Summary of Investments [Abstract]    
Cost [1] 902  
Gross unrealized gains 7  
Gross unrealized losses 0  
Estimated fair value 909  
Equity Securities [Member]    
Summary of Investments [Abstract]    
Cost 19,731 14,712
Gross unrealized gains 9,318 0
Gross unrealized losses (2,322) (870)
Estimated fair value 26,727 13,842
Equity Securities in Public Company [Member]    
Summary of Investments [Abstract]    
Cost [3] 4,712 4,712
Gross unrealized gains 0 0
Gross unrealized losses (2,322) (870)
Estimated fair value 2,390 3,842
Equity Securities in Private Companies [Member]    
Summary of Investments [Abstract]    
Cost [4] 15,019 10,000
Gross unrealized gains 9,318 0
Gross unrealized losses 0 0
Estimated fair value $ 24,337 $ 10,000
[1] We hold our available-for-sale securities at amortized cost.
[2] Includes investments classified as cash equivalents on our condensed consolidated balance sheet.
[3] Our equity securities included in other current assets consisted of our investments in publicly-traded companies. We recognize publicly-traded equity securities at fair value.
[4] Our equity securities included in deposits and other assets consisted of our investments in privately-held companies. We recognize our private company equity securities at cost minus impairments, plus or minus changes resulting from observable price changes in orderly transactions for the identical or similar investment of the same issuer on our condensed consolidated balance sheet.