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Derivative Financial Instruments (Narrative) (Details) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Derivative [Line Items]        
Gains or losses reclassified into earnings $ 0 $ 0 $ 0 $ 0
Maximum length of time over which exposure was being hedged     23 months  
Net investment gains (losses) recognized in AOCI $ 37,500,000 (62,300,000) $ 120,900,000 (194,700,000)
Foreign Currency Denominate        
Derivative [Line Items]        
Estimated net gain (loss) to be reclassified into net income within the next twelve months relating to cash flow hedges     3,300,000  
Foreign Currency Denominated Debt Securities        
Derivative [Line Items]        
Estimated net gain (loss) to be reclassified into net income within the next twelve months relating to cash flow hedges     300,000  
Available for Sale Debt Securities        
Derivative [Line Items]        
Estimated net gain (loss) to be reclassified into net income within the next twelve months relating to cash flow hedges     $ (1,100,000)  
Cash Flow Hedges        
Derivative [Line Items]        
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net   $ 0   $ 0