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Fair Value Measurements - Narrative (Details) - USD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Percent of debt and equity securities classified in Level 2 96.00%    
Maximum percentage of investments classified in Level 2 representing foreign bonds priced using unadjusted broker quotes 1.00%    
Percent of debt and equity securities classified in Level 3 3.00%    
Realized investment losses on assets measured at fair value under certain conditions, after-tax $ 0 $ 0  
Realized investment losses on equity securities with no readily determinable fair value $ 0
Fair value of off-balance-sheet financial assets  
Fair value of off-balance-sheet financial liabilities  
Maximum      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Assets measured at fair value under certain conditions as a percent of total investments 1.00% 1.00%  
Separate Account Assets      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Activity, including transfers in and out of Level 3  
Separate Account Assets | Recurring | Securities partnerships      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Expected liquidation period after inception 10 years    
Other Derivatives | Significant Other Observable Inputs (Level 2)      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Adjustment for credit risk on derivatives assets $ 0   0
Adjustment for credit risk on derivatives liabilities $ 0   $ 0