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Investments - Corporate Debt Securities by Industry Group (Details)
Mar. 31, 2020
Dec. 31, 2019
Debt Securities By Industry [Line Items]    
Corporate debt securities as component of total (percent) 100.00% 100.00%
Financial    
Debt Securities By Industry [Line Items]    
Corporate debt securities as component of total (percent) 40.00% 38.00%
Consumer    
Debt Securities By Industry [Line Items]    
Corporate debt securities as component of total (percent) 23.00% 26.00%
Communications    
Debt Securities By Industry [Line Items]    
Corporate debt securities as component of total (percent) 11.00% 10.00%
Utilities    
Debt Securities By Industry [Line Items]    
Corporate debt securities as component of total (percent) 10.00% 9.00%
Industrial    
Debt Securities By Industry [Line Items]    
Corporate debt securities as component of total (percent) 9.00% 8.00%
Technology    
Debt Securities By Industry [Line Items]    
Corporate debt securities as component of total (percent) 7.00% 7.00%
Energy    
Debt Securities By Industry [Line Items]    
Corporate debt securities as component of total (percent) 0.00% 2.00%