v3.26.3
Debt - Narrative (Details) - USD ($)
$ in Thousands
 1 Months Ended 9 Months Ended
Mar. 31, 2026
Sep. 30, 2026
Jun. 30, 2026
Jun. 15, 2026
Senior Notes | 6.150% Senior Notes due 2037        
Schedule of Long Term and Short Term Debt Instruments [Line Items]        
Debt face amount       $ 750,000
Stated interest rate   6.15%   6.15%
Senior Notes | 5.170% Senior Notes due 2027        
Schedule of Long Term and Short Term Debt Instruments [Line Items]        
Debt face amount   $ 400,000    
Stated interest rate   5.17%    
Surplus Notes | 2027 Surplus Notes        
Schedule of Long Term and Short Term Debt Instruments [Line Items]        
Debt face amount   $ 250,000    
Stated interest rate   8.20%    
Line of Credit | Senior Notes 2036 And 2056 | Unsecured Debt        
Schedule of Long Term and Short Term Debt Instruments [Line Items]        
Stockholders' equity threshold triggering required facility exercise $ 2,800,000      
Origination costs   $ 9,000    
Line of Credit | 2036 Senior Notes | Unsecured Debt        
Schedule of Long Term and Short Term Debt Instruments [Line Items]        
Stated interest rate 6.311%      
Debt instrument, term 10 years      
Pre-capitalized securities, sold by trust $ 500,000      
Debt maximum issuable amount $ 500,000      
Fee percentage 2.066%      
Line of Credit | 2056 Senior Notes | Unsecured Debt        
Schedule of Long Term and Short Term Debt Instruments [Line Items]        
Stated interest rate 7.28%      
Debt instrument, term 30 years      
Pre-capitalized securities, sold by trust $ 400,000      
Debt maximum issuable amount $ 400,000      
Fee percentage 2.43%      
Line of Credit | The 2026 Revolving Credit Facility | Revolving Credit Facility        
Schedule of Long Term and Short Term Debt Instruments [Line Items]        
Maximum borrowing capacity     $ 1,250,000  
Line of credit facility, increase limit   $ 500,000    
Line of credit facility, option to extend, period (up to)   2 years    
Consolidated net worth percentage   65.00%    
Net worth percentage, increase adjustment from equity issuances   50.00%    
Ratio of indebtedness to net capital   0.35    
Line of Credit | The 2026 Revolving Credit Facility | Letter of Credit        
Schedule of Long Term and Short Term Debt Instruments [Line Items]        
Maximum borrowing capacity   $ 500,000    
Line of Credit | The Revolving Credit Facility Terminating February 2028 | Revolving Credit Facility        
Schedule of Long Term and Short Term Debt Instruments [Line Items]        
Line of credit facility, terminated     $ 1,000,000  
Line of Credit | Uncommitted Money Market Line Credit Agreement        
Schedule of Long Term and Short Term Debt Instruments [Line Items]        
Maximum borrowing capacity   500,000    
Minimum cash advance request   $ 100