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Summary of Significant Accounting Policies (Reconciliation of Impact of Adopting ASU 2018-12 on OCI, Prior Period) (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2023
Sep. 30, 2022
Accounting Policies [Abstract]        
Other Comprehensive Income (Loss), Net of Tax $ (150,019) $ (407,663) $ (123,257) $ (1,169,684)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Other Comprehensive Income (Loss), Net of Tax $ (150,019) (407,663) $ (123,257) (1,169,684)
Effect of Modified Retrospective Application Accounting Standards Update 2018-12        
Accounting Policies [Abstract]        
Reverse benefit included in change in net unrealized gains (losses) on available-for-sale-investments, net of taxes, related to reversal of previously recognized reserve deficiency adjustment   0   (44,682)
Change in discount rate for life and annuity benefits, net of taxes   25,244   149,135
Other Comprehensive Income (Loss), Net of Tax   25,244   104,453
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Reverse benefit included in change in net unrealized gains (losses) on available-for-sale-investments, net of taxes, related to reversal of previously recognized reserve deficiency adjustment   0   (44,682)
Change in discount rate for life and annuity benefits, net of taxes   25,244   149,135
Other Comprehensive Income (Loss), Net of Tax   $ 25,244   $ 104,453