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Available for Sale Securities (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2022
USD ($)
Dec. 31, 2021
USD ($)
Dec. 31, 2020
USD ($)
Available for Sale Securities [Line Items]      
Available for sale securities, Amortized Cost $ 12,359,413 $ 13,064,436  
Available for sale securities 11,493,860 13,157,817  
Available for sale securities, Gross Unrealized Gain 28,661 207,361  
Available for sale securities, Gross Unrealized Loss (894,214) (113,980)  
Available for Sale Securities, Realized Gain (Loss) [Abstract]      
Proceeds from sales of available for sale securities 307,481 622,881 $ 384,507
Available for sale securities, Gross realized gains 5,054 5,702 9,976
Available for sale securities, Gross realized losses (6,025) (1,998) (66)
Available for sale securities, Related federal and state income tax expense (227) 889 $ 2,524
Asset Pledged as Collateral with Right      
Available for Sale Securities, Realized Gain (Loss) [Abstract]      
Pledged Securities 11,200,000 10,200,000  
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 121,279    
Available for sale securities, Debt Maturities, One to Five Years, Amortized Cost 2,529,961    
Available for sale securities, Debt Maturities, Six to Ten Years, Amortized Cost 2,396,317    
Available for sale securities, Debt Maturities, Over Ten Years, Amortized Cost 534,075    
Available for sale securities, Maturity, Allocated and Single Maturity Date, Amortized Cost 5,581,632    
Available for Sale Securities, Maturity, Fair Value, Rolling Maturity [Abstract]      
Available for sale securities, Debt Maturities, Less Than One Year, Fair Value 118,907    
Available for sale securities, Debt Maturities, One to Five Years, Fair Value 2,358,353    
Available for sale securities, Debt Maturities, Six to Ten Years, Fair Value 2,126,561    
Available for sale securities, Debt Maturities, Over Ten Years, Fair Value 497,967    
Available for sale securities, Debt Maturities, Single Maturity Date, Fair value $ 5,101,788    
Available for sale securities, Debt Maturities, Weighted Average Maturity [1] 6.11    
U.S. Treasury [Member]      
Available for Sale Securities [Line Items]      
Available for sale securities, Amortized Cost $ 1,000 1,001  
Available for sale securities 898 1,000  
Available for sale securities, Gross Unrealized Gain 0 0  
Available for sale securities, Gross Unrealized Loss (102) (1)  
Municipal securities [Member]      
Available for Sale Securities [Line Items]      
Available for sale securities, Amortized Cost 687,875 515,551  
Available for sale securities 624,500 508,365  
Available for sale securities, Gross Unrealized Gain 321 1,302  
Available for sale securities, Gross Unrealized Loss (63,696) (8,488)  
Commercial agency mortgage-backed securities [Member]      
Available for Sale Securities [Line Items]      
Available for sale securities, Amortized Cost 4,892,257 4,628,172  
Available for sale securities 4,475,917 4,617,025  
Available for sale securities, Gross Unrealized Gain 3,479 36,868  
Available for sale securities, Gross Unrealized Loss (419,819) (48,015)  
Other debt securities [Member]      
Available for Sale Securities [Line Items]      
Available for sale securities, Amortized Cost 500 500  
Available for sale securities 473 472  
Available for sale securities, Gross Unrealized Gain 0 0  
Available for sale securities, Gross Unrealized Loss (27) (28)  
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] 6,777,781    
Available for Sale Securities, Maturity, Fair Value, Rolling Maturity [Abstract]      
Available for sale securities, Debt Maturities, without Single Maturity Date, Fair value $ 6,392,072    
Available for sale securities, Debt Maturities, Average Expected Life of Mortgage-backed Securities 4 years 4 months 24 days    
Residential agency mortgage-backed securities [Member]      
Available for Sale Securities [Line Items]      
Available for sale securities, Amortized Cost $ 6,161,358 7,908,587  
Available for sale securities 5,814,496 8,006,616  
Available for sale securities, Gross Unrealized Gain 13,085 155,477  
Available for sale securities, Gross Unrealized Loss (359,947) (57,448)  
Residential non-agency mortgage-backed securities [Member]      
Available for Sale Securities [Line Items]      
Available for sale securities, Amortized Cost 616,423 10,625  
Available for sale securities 577,576 24,339  
Available for sale securities, Gross Unrealized Gain 11,776 13,714  
Available for sale securities, Gross Unrealized Loss $ (50,623) $ 0  
[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 4.4 years based upon current prepayment assumptions.