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Derivative Instruments and Hedging Activities (Narrative) (Details) - USD ($)
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Dec. 31, 2020
Derivative [Line Items]      
Unrealized gains, before tax $ (29,057,000) $ 108,639,000  
Unrealized gain, after tax (21,183,000) 80,501,000  
Income recognized 1,600,000    
Cash flow hedge gain (loss) to be reclassified 43,000,000    
Cash flow hedge termination gain 13,000,000    
Collateral requirements 121,700,000   $ 155,400,000
Variation margin, amount reducing derivative asset 81,800,000   162,700,000
Foreign Currency Exchange Forwards      
Derivative [Line Items]      
Notional amount 18,400,000   $ 24,100,000
Cash Flow Hedges      
Derivative [Line Items]      
Unrealized gains, before tax 757,000 9,800,000  
Unrealized gain, after tax $ 565,000 $ 7,300,000