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Debt - Senior Unsecured Notes (Details) - USD ($)
$ in Millions
1 Months Ended
Sep. 18, 2023
Jun. 22, 2023
May 04, 2021
Apr. 12, 2021
Apr. 30, 2021
Oct. 28, 2023
Jan. 28, 2023
Jun. 22, 2018
Debt Instrument [Line Items]                
Debt outstanding           $ 4,185.9 $ 4,492.1  
Senior Notes                
Debt Instrument [Line Items]                
Debt outstanding           $ 2,000.0    
Redemption price percentage 101.00%     101.00% 101.00%      
Senior Notes | 5.750% 2029 Senior Notes                
Debt Instrument [Line Items]                
Aggregate principal amount $ 500.0              
Stated interest rate 5.75%         5.75%    
Effective interest rate 5.891%              
Senior Notes | 5.950% 2033 Senior Notes                
Debt Instrument [Line Items]                
Aggregate principal amount $ 500.0              
Stated interest rate 5.95%         5.95%    
Effective interest rate 6.082%              
Senior Notes | Senior Notes Due 2029 and 2033                
Debt Instrument [Line Items]                
Debt outstanding           $ 1,000.0    
Senior Notes | 1.650% 2026 Senior Notes                
Debt Instrument [Line Items]                
Aggregate principal amount       $ 500.0        
Stated interest rate       1.65%   1.65%    
Effective interest rate       1.839%        
Senior Notes | 2.450% 2028 Senior Notes                
Debt Instrument [Line Items]                
Aggregate principal amount       $ 750.0        
Stated interest rate       2.45%   2.45%    
Effective interest rate       2.554%        
Senior Notes | 2.950% 2031 Senior Notes                
Debt Instrument [Line Items]                
Aggregate principal amount       $ 750.0        
Stated interest rate       2.95%   2.95%    
Effective interest rate       3.043%        
Senior Notes | MTG/MTI Senior Notes                
Debt Instrument [Line Items]                
Debt outstanding           $ 499.9    
Senior Notes | MTG Senior Notes                
Debt Instrument [Line Items]                
Debt outstanding         $ 1,000.0      
Senior Notes | MTG Senior Notes Due 2023                
Debt Instrument [Line Items]                
Aggregate principal amount               $ 500.0
Stated interest rate               4.20%
Principal amount exchanged     $ 433.9          
Senior Notes | MTG Senior Notes Due 2028                
Debt Instrument [Line Items]                
Aggregate principal amount         $ 500.0     $ 500.0
Stated interest rate         4.875%     4.875%
Effective interest rate         4.94%      
Principal amount exchanged     $ 479.5          
Senior Notes | MTI Senior Notes Due 2023                
Debt Instrument [Line Items]                
Aggregate principal amount         $ 500.0      
Stated interest rate         4.20%      
Senior Notes | MTI Senior Notes Due 2028                
Debt Instrument [Line Items]                
Stated interest rate         4.875%      
Effective interest rate         4.988%      
Senior Notes | MTI and MTG 2023 Notes                
Debt Instrument [Line Items]                
Repayments of senior notes   $ 500.0