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Fair Value Measurements (Details) - USD ($)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) $ 1,200,000 $ 0 $ 0
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount 0 0  
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount 0 0  
Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount 0 0  
Fair Value, Liabilities, Level 2 to Level 1 Transfers, Amount 0 0  
Equity Securities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Trading Securities, Change in Unrealized Holding Gain (Loss) 0 0 0
Total debt securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Trading Securities, Change in Unrealized Holding Gain (Loss) $ 10,500,000 $ 10,000,000.0 $ (6,700,000)