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Available for Sale Securities (Details)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2024
USD ($)
Sep. 30, 2023
USD ($)
Sep. 30, 2024
USD ($)
Sep. 30, 2023
USD ($)
Dec. 31, 2023
USD ($)
Debt Securities, Available-for-sale [Line Items]          
Available-for-sale Securities, Amortized Cost $ 13,323,346   $ 13,323,346   $ 12,903,305
Available-for-sale Securities, Fair Value 13,015,986   13,015,986   12,286,681
Available-for-sale Securities, Gross Unrealized Gain 117,579   117,579   52,821
Available-for-sale Securities, Gross Unrealized Loss (424,939)   (424,939)   (669,445)
Debt Securities, Available-for-sale, Realized Gain (Loss) [Abstract]          
Available-for-sale Securities, Proceeds 101,522 $ 0 839,352 $ 135,489  
Available-for-sale Securities, Gross realized gains 1,802 0 2,257 703  
Available-for-sale Securities, Gross realized losses (2,493) 0 (48,085) (3,713)  
Available-for-sale Securities, Related federal and state income tax expense (benefit) (163) $ 0 (10,779) $ (708)  
Asset Pledged as Collateral with Right          
Debt Securities, Available-for-sale, Realized Gain (Loss) [Abstract]          
Pledged Securities 10,500,000   10,500,000   13,900,000
Fixed Maturity Securities [Member]          
Debt Securities, Available-for-sale, Maturity, Amortized Cost, Rolling Maturity [Abstract]          
Available-for-sale Securities, Debt Maturities, Less than One Year, Amortized Cost Basis 317,443   317,443    
Available-for-sale Securities, Debt Maturities, One to Five Years, Amortized Cost Basis 2,432,178   2,432,178    
Available-for-sale Securities, Debt Maturities, Six To Ten Years, Amortized Cost Basis 631,714   631,714    
Available-for-sale Securities, Debt Maturities, Over Ten Years, Amortized Cost Basis 436,923   436,923    
Available-for-sale Securities, Debt Maturities, Allocated and Single Maturity Date, Amortized Cost 3,818,258   3,818,258    
Debt Securities, Available-for-sale, Maturity, Fair Value, Rolling Maturity [Abstract]          
Available-for-sale Securities, Debt Maturities, Less than One Year, Fair Value 314,948   314,948    
Available-for-sale Securities, Debt Maturities, One to Five Years, Fair Value 2,274,047   2,274,047    
Available-for-sale Securities, Debt Maturities, Six to Ten Years, Fair Value 585,305   585,305    
Available-for-sale Securities, Debt Maturities, Over Ten Years, Fair Value 424,975   424,975    
Available-for-sale Securities, Debt Maturities, Allocated and Single Maturity Date, Fair Value $ 3,599,275   $ 3,599,275    
Available-for-sale Securities, Debt Maturities, Weighted Average Maturity [1] 5.10   5.10    
U.S. Treasury [Member]          
Debt Securities, Available-for-sale [Line Items]          
Available-for-sale Securities, Amortized Cost $ 1,000   $ 1,000   1,000
Available-for-sale Securities, Fair Value 951   951   925
Available-for-sale Securities, Gross Unrealized Gain 0   0   0
Available-for-sale Securities, Gross Unrealized Loss (49)   (49)   (75)
Municipal securities [Member]          
Debt Securities, Available-for-sale [Line Items]          
Available-for-sale Securities, Amortized Cost 241,447   241,447   544,707
Available-for-sale Securities, Fair Value 228,171   228,171   502,833
Available-for-sale Securities, Gross Unrealized Gain 7   7   1
Available-for-sale Securities, Gross Unrealized Loss (13,283)   (13,283)   (41,875)
Commercial agency mortgage-backed securities [Member]          
Debt Securities, Available-for-sale [Line Items]          
Available-for-sale Securities, Amortized Cost 3,575,311   3,575,311   4,456,918
Available-for-sale Securities, Fair Value 3,369,680   3,369,680   4,147,853
Available-for-sale Securities, Gross Unrealized Gain 3,857   3,857   2,972
Available-for-sale Securities, Gross Unrealized Loss (209,488)   (209,488)   (312,037)
Other debt securities [Member]          
Debt Securities, Available-for-sale [Line Items]          
Available-for-sale Securities, Amortized Cost 500   500   500
Available-for-sale Securities, Fair Value 473   473   473
Available-for-sale Securities, Gross Unrealized Gain 0   0   0
Available-for-sale Securities, Gross Unrealized Loss (27)   (27)   (27)
Residential Mortgage Backed Securities [Member]          
Debt Securities, Available-for-sale, Maturity, Amortized Cost, Rolling Maturity [Abstract]          
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Amortized Cost [2] 9,505,088   9,505,088    
Debt Securities, Available-for-sale, Maturity, Fair Value, Rolling Maturity [Abstract]          
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Fair Value $ 9,416,711   9,416,711    
Available-for-sale Securities, Debt Maturities, Average Expected Life of Mortgage-backed Securities 3 years 10 months 24 days        
Residential agency mortgage-backed securities [Member]          
Debt Securities, Available-for-sale [Line Items]          
Available-for-sale Securities, Amortized Cost $ 8,649,078   8,649,078   7,066,645
Available-for-sale Securities, Fair Value 8,577,928   8,577,928   6,834,720
Available-for-sale Securities, Gross Unrealized Gain 98,405   98,405   36,983
Available-for-sale Securities, Gross Unrealized Loss (169,555)   (169,555)   (268,908)
Residential non-agency mortgage-backed securities [Member]          
Debt Securities, Available-for-sale [Line Items]          
Available-for-sale Securities, Amortized Cost 856,010   856,010   833,535
Available-for-sale Securities, Fair Value 838,783   838,783   799,877
Available-for-sale Securities, Gross Unrealized Gain 15,310   15,310   12,865
Available-for-sale Securities, Gross Unrealized Loss $ (32,537)   $ (32,537)   $ (46,523)
[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 3.9 years based upon current prepayment assumptions.