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Investments, Summary of Investments (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Summary of Investments [Abstract]    
Cost $ 1,533,544 $ 1,846,074
Gross unrealized gains 24,515 5,007
Gross unrealized losses (2,821) (1,166)
Estimated fair value 1,555,238 1,849,915
Available-for-sale Securities [Member]    
Summary of Investments [Abstract]    
Cost [1] 1,513,770 1,831,362
Gross unrealized gains 8,577 5,007
Gross unrealized losses (140) (296)
Estimated fair value 1,522,207 1,836,073
Available-for-sale Securities [Member] | Securities with Maturity of One Year or Less [Member]    
Summary of Investments [Abstract]    
Cost [1] 1,025,454 1,206,616
Gross unrealized gains 2,977 2,012
Gross unrealized losses (71) (118)
Estimated fair value 1,028,360 1,208,510
Available-for-sale Securities [Member] | Securities with Maturity of More than One Year [Member]    
Summary of Investments [Abstract]    
Cost [1] 488,316 624,746
Gross unrealized gains 5,600 2,995
Gross unrealized losses (69) (178)
Estimated fair value 493,847 627,563
Corporate Debt Securities [Member] | Securities with Maturity of One Year or Less [Member]    
Summary of Investments [Abstract]    
Cost [1],[2] 514,182 669,665
Gross unrealized gains 2,194 1,451
Gross unrealized losses (41) (43)
Estimated fair value 516,335 671,073
Corporate Debt Securities [Member] | Securities with Maturity of More than One Year [Member]    
Summary of Investments [Abstract]    
Cost [1] 325,079 428,627
Gross unrealized gains 4,941 2,911
Gross unrealized losses (40) (43)
Estimated fair value 329,980 431,495
Debt Securities issued by U.S. Government Agencies [Member] | Securities with Maturity of One Year or Less [Member]    
Summary of Investments [Abstract]    
Cost [1] 94,234 188,216
Gross unrealized gains 354 303
Gross unrealized losses (2) (43)
Estimated fair value 94,586 188,476
Debt Securities issued by U.S. Government Agencies [Member] | Securities with Maturity of More than One Year [Member]    
Summary of Investments [Abstract]    
Cost [1] 80,099 140,988
Gross unrealized gains 185 57
Gross unrealized losses (9) (117)
Estimated fair value 80,275 140,928
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] 307,576 327,670
Gross unrealized gains 233 232
Gross unrealized losses (9) (27)
Estimated fair value 307,800 327,875
Debt Securities issued by the U.S. Treasury [Member] | Securities with Maturity of More than One Year [Member]    
Summary of Investments [Abstract]    
Cost [1] 50,318 35,822
Gross unrealized gains 383 9
Gross unrealized losses (4) (12)
Estimated fair value 50,697 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] 104,271 21,065
Gross unrealized gains 196 26
Gross unrealized losses (12) (5)
Estimated fair value 104,455 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] 31,779 19,309
Gross unrealized gains 91 18
Gross unrealized losses (16) (6)
Estimated fair value 31,854 19,321
Other Municipal Debt Securities [Member] | Securities with Maturity of One Year or Less [Member]    
Summary of Investments [Abstract]    
Cost [1] 5,191  
Gross unrealized gains 0  
Gross unrealized losses (7)  
Estimated fair value 5,184  
Other Municipal Debt Securities [Member] | Securities with Maturity of More than One Year [Member]    
Summary of Investments [Abstract]    
Cost [1] 1,041  
Gross unrealized gains 0  
Gross unrealized losses 0  
Estimated fair value 1,041  
Equity Securities [Member]    
Summary of Investments [Abstract]    
Cost 19,774 14,712
Gross unrealized gains 15,938 0
Gross unrealized losses (2,681) (870)
Estimated fair value 33,031 13,842
Equity Securities in Publicly Traded Company [Member]    
Summary of Investments [Abstract]    
Cost [3] 4,712 4,712
Gross unrealized gains 0 0
Gross unrealized losses (2,681) (870)
Estimated fair value 2,031 3,842
Equity Securities in Private Companies [Member]    
Summary of Investments [Abstract]    
Cost [4] 15,062 10,000
Gross unrealized gains 15,938 0 [4]
Gross unrealized losses 0 0 [4]
Estimated fair value $ 31,000 $ 10,000 [4]
[1] We hold our available-for-sale securities at amortized cost.
[2] Includes investments classified as cash equivalents on our 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 consolidated balance sheet.