Investments - Corporate Debt Securities by Industry Group (Details)
Dec. 31, 2019
Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]    
Corporate debt securities as component of total (percent) 100.00% 100.00%
Financial    
Debt Securities, Available-for-sale [Line Items]    
Corporate debt securities as component of total (percent) 38.00% 38.00%
Consumer    
Debt Securities, Available-for-sale [Line Items]    
Corporate debt securities as component of total (percent) 26.00% 27.00%
Communications    
Debt Securities, Available-for-sale [Line Items]    
Corporate debt securities as component of total (percent) 10.00% 12.00%
Utilities    
Debt Securities, Available-for-sale [Line Items]    
Corporate debt securities as component of total (percent) 9.00% 7.00%
Industrial    
Debt Securities, Available-for-sale [Line Items]    
Corporate debt securities as component of total (percent) 8.00% 7.00%
Technology    
Debt Securities, Available-for-sale [Line Items]    
Corporate debt securities as component of total (percent) 7.00% 6.00%
Energy    
Debt Securities, Available-for-sale [Line Items]    
Corporate debt securities as component of total (percent) 2.00% 2.00%
Other    
Debt Securities, Available-for-sale [Line Items]    
Corporate debt securities as component of total (percent) 0.00% 1.00%