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Available for Sale Securities (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2023
USD ($)
Dec. 31, 2022
USD ($)
Dec. 31, 2021
USD ($)
Available for Sale Securities [Line Items]      
Available for sale securities, Amortized Cost $ 12,903,305 $ 12,359,413  
Available for sale securities 12,286,681 11,493,860  
Available for sale securities, Gross Unrealized Gain 52,821 28,661  
Available for sale securities, Gross Unrealized Loss (669,445) (894,214)  
Available for Sale Securities, Realized Gain (Loss) [Abstract]      
Proceeds from sales of available for sale securities 834,704 307,481 $ 622,881
Available for sale securities, Gross realized gains 1,180 5,054 5,702
Available for sale securities, Gross realized losses (31,816) (6,025) (1,998)
Available for sale securities, Related federal and state income tax expense (7,206) (227) $ 889
Asset Pledged as Collateral with Right      
Available for Sale Securities, Realized Gain (Loss) [Abstract]      
Pledged Securities 13,900,000 11,200,000  
Fixed maturity securities [Member]      
Available for Sale Securities, Debt Maturities, Amortized Cost, Rolling Maturity [Abstract]      
Available for sale securities, Debt Maturities, Less than One Year, Amortized Cost 397,083    
Available for sale securities, Debt Maturities, One to Five Years, Amortized Cost 3,008,053    
Available for sale securities, Debt Maturities, Six to Ten Years, Amortized Cost 1,122,067    
Available for sale securities, Debt Maturities, Over Ten Years, Amortized Cost 475,922    
Available for sale securities, Maturity, Allocated and Single Maturity Date, Amortized Cost 5,003,125    
Available for Sale Securities, Maturity, Fair Value, Rolling Maturity [Abstract]      
Available for sale securities, Debt Maturities, Less Than One Year, Fair Value 392,254    
Available for sale securities, Debt Maturities, One to Five Years, Fair Value 2,764,659    
Available for sale securities, Debt Maturities, Six to Ten Years, Fair Value 1,040,314    
Available for sale securities, Debt Maturities, Over Ten Years, Fair Value 454,857    
Available for sale securities, Debt Maturities, Single Maturity Date, Fair value $ 4,652,084    
Available for sale securities, Debt Maturities, Weighted Average Maturity [1] 5.28    
U.S. Treasury [Member]      
Available for Sale Securities [Line Items]      
Available for sale securities, Amortized Cost $ 1,000 1,000  
Available for sale securities 925 898  
Available for sale securities, Gross Unrealized Gain 0 0  
Available for sale securities, Gross Unrealized Loss (75) (102)  
Municipal securities [Member]      
Available for Sale Securities [Line Items]      
Available for sale securities, Amortized Cost 544,707 687,875  
Available for sale securities 502,833 624,500  
Available for sale securities, Gross Unrealized Gain 1 321  
Available for sale securities, Gross Unrealized Loss (41,875) (63,696)  
Commercial agency mortgage-backed securities [Member]      
Available for Sale Securities [Line Items]      
Available for sale securities, Amortized Cost 4,456,918 4,892,257  
Available for sale securities 4,147,853 4,475,917  
Available for sale securities, Gross Unrealized Gain 2,972 3,479  
Available for sale securities, Gross Unrealized Loss (312,037) (419,819)  
Other debt securities [Member]      
Available for Sale Securities [Line Items]      
Available for sale securities, Amortized Cost 500 500  
Available for sale securities 473 473  
Available for sale securities, Gross Unrealized Gain 0 0  
Available for sale securities, Gross Unrealized Loss (27) (27)  
Residential Mortgage Backed Securities [Member]      
Available for Sale Securities, Debt Maturities, Amortized Cost, Rolling Maturity [Abstract]      
Available for sale securities, Maturity, without Single Maturity Date, Amortized Cost [2] 7,900,180    
Available for Sale Securities, Maturity, Fair Value, Rolling Maturity [Abstract]      
Available for sale securities, Debt Maturities, without Single Maturity Date, Fair value $ 7,634,597    
Available for sale securities, Debt Maturities, Average Expected Life of Mortgage-backed Securities 4 years    
Residential agency mortgage-backed securities [Member]      
Available for Sale Securities [Line Items]      
Available for sale securities, Amortized Cost $ 7,066,645 6,161,358  
Available for sale securities 6,834,720 5,814,496  
Available for sale securities, Gross Unrealized Gain 36,983 13,085  
Available for sale securities, Gross Unrealized Loss (268,908) (359,947)  
Residential non-agency mortgage-backed securities [Member]      
Available for Sale Securities [Line Items]      
Available for sale securities, Amortized Cost 833,535 616,423  
Available for sale securities 799,877 577,576  
Available for sale securities, Gross Unrealized Gain 12,865 11,776  
Available for sale securities, Gross Unrealized Loss $ (46,523) $ (50,623)  
[1] Expected maturities may differ from contractual maturities, because borrowers may have the right to call or prepay obligations with or without penalty.
[2] The average expected lives of residential mortgage-backed securities were 4.0 years based upon current prepayment assumptions.