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Investments, Summary of Investments (Details) - USD ($)
$ in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2020
Dec. 31, 2019
Summary of Investments [Abstract]    
Cost $ 1,727,379 $ 1,846,074
Gross unrealized gains 21,297 5,007
Gross unrealized losses (2,735) (1,166)
Estimated fair value 1,745,941 1,849,915
Available-for-sale Securities [Member]    
Summary of Investments [Abstract]    
Cost [1] 1,707,636 1,831,362
Gross unrealized gains 11,979 5,007
Gross unrealized losses (170) (296)
Estimated fair value 1,719,445 1,836,073
Available-for-sale Securities [Member] | Securities with Maturity of One Year or Less [Member]    
Summary of Investments [Abstract]    
Cost [1] 1,129,208 1,206,616
Gross unrealized gains 4,563 2,012
Gross unrealized losses (22) (118)
Estimated fair value 1,133,749 1,208,510
Available-for-sale Securities [Member] | Securities with Maturity of More than One Year [Member]    
Summary of Investments [Abstract]    
Cost [1] 578,428 624,746
Gross unrealized gains 7,416 2,995
Gross unrealized losses (148) (178)
Estimated fair value 585,696 627,563
Corporate Debt Securities [Member] | Securities with Maturity of One Year or Less [Member]    
Summary of Investments [Abstract]    
Cost [1],[2] 568,857 669,665
Gross unrealized gains 3,443 1,451
Gross unrealized losses (12) (43)
Estimated fair value 572,288 671,073
Corporate Debt Securities [Member] | Securities with Maturity of More than One Year [Member]    
Summary of Investments [Abstract]    
Cost [1] 392,836 428,627
Gross unrealized gains 6,315 2,911
Gross unrealized losses (87) (43)
Estimated fair value 399,064 431,495
Debt Securities issued by U.S. Government Agencies [Member] | Securities with Maturity of One Year or Less [Member]    
Summary of Investments [Abstract]    
Cost [1] 104,329 188,216 [2]
Gross unrealized gains 391 303
Gross unrealized losses (1) (43)
Estimated fair value 104,719 188,476
Debt Securities issued by U.S. Government Agencies [Member] | Securities with Maturity of More than One Year [Member]    
Summary of Investments [Abstract]    
Cost [1] 120,065 140,988
Gross unrealized gains 440 57
Gross unrealized losses (31) (117)
Estimated fair value 120,474 140,928
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] 358,758 327,670
Gross unrealized gains 485 232
Gross unrealized losses (7) (27)
Estimated fair value 359,236 327,875
Debt Securities issued by the U.S. Treasury [Member] | Securities with Maturity of More than One Year [Member]    
Summary of Investments [Abstract]    
Cost [1] 31,598 35,822
Gross unrealized gains 424 9
Gross unrealized losses (6) (12)
Estimated fair value 32,016 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] 96,363 21,065 [2]
Gross unrealized gains 238 26
Gross unrealized losses (2) (5)
Estimated fair value 96,599 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] 27,637 19,309
Gross unrealized gains 237 18
Gross unrealized losses (1) (6)
Estimated fair value 27,873 19,321
Other Municipal Debt Securities [Member] | Securities with Maturity of One Year or Less [Member]    
Summary of Investments [Abstract]    
Cost [1] 901  
Gross unrealized gains 6  
Gross unrealized losses 0  
Estimated fair value 907  
Equity Securities [Member]    
Summary of Investments [Abstract]    
Cost 19,743 14,712
Gross unrealized gains 9,318 0
Gross unrealized losses (2,565) (870)
Estimated fair value 26,496 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 (2,565) (870)
Estimated fair value 2,147 3,842
Equity Securities in Private Companies [Member]    
Summary of Investments [Abstract]    
Cost [4] 15,031 10,000
Gross unrealized gains 9,318 0
Gross unrealized losses 0 0
Estimated fair value $ 24,349 $ 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.