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Fair Value Measurements (Narrative) (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Investment Maturity Period     1 year   1 year
Net investment gains (losses) $ 425,653 $ 105,249 $ 1,037,844 $ (17,749)  
Equity securities 6,875,933   6,875,933   $ 5,720,945
Fair Value, 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 6,875,933   6,875,933   5,720,945
Fair Value, 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 0 0  
Liabilities measured at fair value on a nonrecurring basis 0 0 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) (1,600) $ (2,600) (2,100) $ (8,500)  
Equity securities $ 2,400   $ 2,400   $ 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