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Available for Sale Securities (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Available for Sale Securities [Line Items]      
Available for sale securities, Amortized Cost $ 12,609,850 $ 11,131,494  
Available for sale securities 13,050,665 11,269,643  
Available for sale securities, Gross Unrealized Gain 449,654 158,435  
Available for sale securities, Gross Unrealized Loss (8,839) (20,286)  
Available for Sale Securities, Realized Gain (Loss) [Abstract]      
Proceeds from sales of available for sale securities 384,507 1,211,718 $ 745,643
Available for sale securities, Gross realized gains 9,976 14,996 7,117
Available for sale securities, Gross realized losses (66) (9,399) (9,918)
Available for sale securities, Related federal and state income tax expense 2,524 1,425 $ (713)
Available for sale securities, Pledged as Collateral, Fair Value 11,600,000 10,100,000  
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 78,673    
Available for sale securities, Debt Maturities, One to Five Years, Amortized Cost 1,413,405    
Available for sale securities, Debt Maturities, Six to Ten Years, Amortized Cost 1,472,918    
Available for sale securities, Debt Maturities, Over Ten Years, Amortized Cost 608,278    
Available for sale securities, Maturity, Allocated and Single Maturity Date, Amortized Cost 3,573,274    
Available for Sale Securities, Maturity, Fair Value, Rolling Maturity [Abstract]      
Available for sale securities, Debt Maturities, Less Than One Year, Fair Value 78,918    
Available for sale securities, Debt Maturities, One to Five Years, Fair Value 1,469,630    
Available for sale securities, Debt Maturities, Six to Ten Years, Fair Value 1,498,376    
Available for sale securities, Debt Maturities, Over Ten Years, Fair Value 630,500    
Available for sale securities, Debt Maturities, Single Maturity Date, Fair value $ 3,677,424    
Available for sale securities, Debt Maturities, Weighted Average Maturity [1] 7.79    
U.S. Treasury [Member]      
Available for Sale Securities [Line Items]      
Available for sale securities, Amortized Cost $ 500 1,598  
Available for sale securities 508 1,600  
Available for sale securities, Gross Unrealized Gain 8 2  
Available for sale securities, Gross Unrealized Loss 0 0  
Municipal securities [Member]      
Available for Sale Securities [Line Items]      
Available for sale securities, Amortized Cost 165,318 1,789  
Available for sale securities 167,979 1,861  
Available for sale securities, Gross Unrealized Gain 2,666 72  
Available for sale securities, Gross Unrealized Loss (5) 0  
Commercial agency mortgage-backed securities [Member]      
Available for Sale Securities [Line Items]      
Available for sale securities, Amortized Cost 3,406,956 3,145,342  
Available for sale securities 3,508,465 3,178,005  
Available for sale securities, Gross Unrealized Gain 103,590 37,808  
Available for sale securities, Gross Unrealized Loss (2,081) (5,145)  
Other debt securities [Member]      
Available for Sale Securities [Line Items]      
Available for sale securities, Amortized Cost 500 500  
Available for sale securities 472 472  
Available for sale securities, Gross Unrealized Gain 0 0  
Available for sale securities, Gross Unrealized Loss (28) (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] 9,036,576    
Available for Sale Securities, Maturity, Fair Value, Rolling Maturity [Abstract]      
Available for sale securities, Debt Maturities, without Single Maturity Date, Fair value $ 9,373,241    
Available for sale securities, Debt Maturities, Average Expected Life of Mortgage-backed Securities 3.0 years    
Residential agency mortgage-backed securities [Member]      
Available for Sale Securities [Line Items]      
Available for sale securities, Amortized Cost $ 9,019,013 7,956,297  
Available for sale securities 9,340,471 8,046,096  
Available for sale securities, Gross Unrealized Gain 328,183 104,912  
Available for sale securities, Gross Unrealized Loss (6,725) (15,113)  
Residential non-agency mortgage-backed securities [Member]      
Available for Sale Securities [Line Items]      
Available for sale securities, Amortized Cost 17,563 25,968  
Available for sale securities 32,770 41,609  
Available for sale securities, Gross Unrealized Gain 15,207 15,641  
Available for sale securities, Gross Unrealized Loss $ 0 $ 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 3.0 years based upon current prepayment assumptions.