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Debt and Equity Securities (Narrative) (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Schedule Of Debt and Equity Securities [Line Items]          
Realized gains on sales of debt securities $ 1,800 $ 700 $ 4,900 $ 1,300  
Realized losses on sales of debt securities 1,700 2,600 4,700 3,800  
Proceeds from sale of debt securities 231,300 186,600 622,700 342,300  
Realized and unrealized net gains (losses) on equity securities 8,900 7,700 41,400 1,800  
Net unrealized gains on equity securities 8,400 $ 5,600 37,400 $ 1,800  
Debt securities, Estimated fair value 5,999,922   5,999,922   $ 5,713,811
Estimated fair value of debt securities, Unrealized loss position 1,741,241   1,741,241   $ 3,373,514
Bank Loans          
Schedule Of Debt and Equity Securities [Line Items]          
Debt securities, Estimated fair value 78,600   78,600    
Estimated fair value of debt securities, Unrealized loss position 58,700   58,700    
Emerging Market Securities          
Schedule Of Debt and Equity Securities [Line Items]          
Debt securities, Estimated fair value 67,100   67,100    
Estimated fair value of debt securities, Unrealized loss position 4,200   4,200    
Non-Investment Grade          
Schedule Of Debt and Equity Securities [Line Items]          
Debt securities, Estimated fair value 118,467   118,467    
Estimated fair value of debt securities, Unrealized loss position 66,441   66,441    
Non-Investment Grade | Bank Loans          
Schedule Of Debt and Equity Securities [Line Items]          
Debt securities, Estimated fair value 68,700   68,700    
Estimated fair value of debt securities, Unrealized loss position 53,000   53,000    
Non-Investment Grade | High Yield Corporate Debt Securities          
Schedule Of Debt and Equity Securities [Line Items]          
Debt securities, Estimated fair value 39,500   39,500    
Estimated fair value of debt securities, Unrealized loss position 11,000   11,000    
Non-Investment Grade | Emerging Market Securities          
Schedule Of Debt and Equity Securities [Line Items]          
Debt securities, Estimated fair value 8,900   8,900    
Estimated fair value of debt securities, Unrealized loss position $ 2,400   $ 2,400