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Fair Value Measurements (Narrative) (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Net investment gains (losses) $ 612,191 $ (122,998)  
Equity securities 6,381,173   $ 5,720,945
Fair Value, Measurements, Recurring [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value, level 1 to level 2 transfers, amount 0 0  
Fair value, level 2 to level 1 transfers, amount 0 0  
Equity securities 6,381,173   5,720,945
Fair Value, Measurements, Nonrecurring [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Assets measured at fair value on a nonrecurring basis 0 0  
Liabilities measured at fair value on a nonrecurring basis 0 0  
CATCo Reinsurance Opportunities Fund Ltd. (CROF) [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Net investment gains (losses) (500) $ (5,900)  
Equity securities $ 4,000   $ 4,500
Maximum [Member] | Short-Term Investments [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investment Maturity Period 1 year