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Investment Securities - Amortized cost and estimated fair value of investment securities (Details) - USD ($)
$ in Thousands
Dec. 31, 2021
Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]    
Amortized cost, current $ 954,797  
Accumulated gross unrealized gain, current 359  
Accumulated gross unrealized loss, current 6,479  
Debt securities, current 948,677 $ 0
Amortized cost, noncurrent 40,394 37,219
Accumulated gross unrealized gain, noncurrent 4 55
Accumulated gross unrealized loss, noncurrent 748 1
Debt securities, noncurrent 39,650 37,273
Amortized Cost 995,191 37,219
Total Unrealized Gains 363 55
Total Unrealized Losses 7,227 1
Fair Value 988,327 37,273
Trading securities 11,300 9,600
U.S. treasury notes    
Debt Securities, Available-for-sale [Line Items]    
Amortized cost, current 308,058  
Accumulated gross unrealized gain, current 250  
Accumulated gross unrealized loss, current 1,113  
Debt securities, current 307,195  
Corporate debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized cost, current 355,102  
Accumulated gross unrealized gain, current 30  
Accumulated gross unrealized loss, current 3,289  
Debt securities, current 351,843  
Municipal bonds    
Debt Securities, Available-for-sale [Line Items]    
Amortized cost, current 31,273  
Accumulated gross unrealized gain, current 44  
Accumulated gross unrealized loss, current 149  
Debt securities, current 31,168  
Amortized cost, noncurrent 3,107 195
Accumulated gross unrealized gain, noncurrent 1 2
Accumulated gross unrealized loss, noncurrent 0 0
Debt securities, noncurrent 3,108 197
Asset-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized cost, current 120,774  
Accumulated gross unrealized gain, current 24  
Accumulated gross unrealized loss, current 587  
Debt securities, current 120,211  
Amortized cost, noncurrent 167 211
Accumulated gross unrealized gain, noncurrent 1 0
Accumulated gross unrealized loss, noncurrent 0 1
Debt securities, noncurrent 168 210
Mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized cost, current 139,590  
Accumulated gross unrealized gain, current 11  
Accumulated gross unrealized loss, current 1,341  
Debt securities, current 138,260  
Amortized cost, noncurrent 121 133
Accumulated gross unrealized gain, noncurrent 2 5
Accumulated gross unrealized loss, noncurrent 0 0
Debt securities, noncurrent 123 138
Mutual fund    
Debt Securities, Available-for-sale [Line Items]    
Amortized cost, noncurrent 27,999 27,680
Accumulated gross unrealized gain, noncurrent 0 48
Accumulated gross unrealized loss, noncurrent 748 0
Debt securities, noncurrent 27,251 27,728
Pooled investment fund    
Debt Securities, Available-for-sale [Line Items]    
Amortized cost, noncurrent 9,000 9,000
Accumulated gross unrealized gain, noncurrent 0 0
Accumulated gross unrealized loss, noncurrent 0 0
Debt securities, noncurrent $ 9,000 $ 9,000