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Available for Sale Securities (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Available for Sale Securities [Line Items]      
Available for sale securities, Amortized Cost $ 11,131,494 $ 8,952,391  
Available for sale securities 11,269,643 8,857,120  
Available for sale securities, Gross Unrealized Gain 158,435 42,986  
Available for sale securities, Gross Unrealized Loss (20,286) (138,257)  
Available for Sale Securities, Realized Gain (Loss) [Abstract]      
Proceeds from sales of available for sale securities 1,211,718 745,643 $ 1,309,215
Available for sale securities, Gross realized gains 14,996 7,117 10,223
Available for sale securities, Gross realized losses (9,399) (9,918) (5,795)
Available for sale securities, Related federal and state income tax expense 1,425 (713) $ 1,722
Available for sale securities, Pledged as Collateral, Fair Value 10,100,000 9,100,000  
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 5,105    
Available for sale securities, Debt Maturities, One to Five Years, Amortized Cost 1,113,463    
Available for sale securities, Debt Maturities, Six to Ten Years, Amortized Cost 1,455,340    
Available for sale securities, Debt Maturities, Over Ten Years, Amortized Cost 575,321    
Available for sale securities, Maturity, Allocated and Single Maturity Date, Amortized Cost 3,149,229    
Available for Sale Securities, Maturity, Fair Value, Rolling Maturity [Abstract]      
Available for sale securities, Debt Maturities, Less Than One Year, Fair Value 5,114    
Available for sale securities, Debt Maturities, One to Five Years, Fair Value 1,122,392    
Available for sale securities, Debt Maturities, Six to Ten Years, Fair Value 1,474,002    
Available for sale securities, Debt Maturities, Over Ten Years, Fair Value 580,430    
Available for sale securities, Debt Maturities, Single Maturity Date, Fair value $ 3,181,938    
Available for sale securities, Debt Maturities, Weighted Average Maturity [1] 8.15    
U.S. Treasury [Member]      
Available for Sale Securities [Line Items]      
Available for sale securities, Amortized Cost $ 1,598 496  
Available for sale securities 1,600 493  
Available for sale securities, Gross Unrealized Gain 2 0  
Available for sale securities, Gross Unrealized Loss 0 (3)  
Municipal and other tax-exempt securities [Member]      
Available for Sale Securities [Line Items]      
Available for sale securities, Amortized Cost 1,789 2,782  
Available for sale securities 1,861 2,864  
Available for sale securities, Gross Unrealized Gain 72 82  
Available for sale securities, Gross Unrealized Loss 0 0  
Commercial agency mortgage-backed securities [Member]      
Available for Sale Securities [Line Items]      
Available for sale securities, Amortized Cost 3,145,342 2,986,297  
Available for sale securities 3,178,005 2,953,889  
Available for sale securities, Gross Unrealized Gain 37,808 7,955  
Available for sale securities, Gross Unrealized Loss (5,145) (40,363)  
Other debt securities [Member]      
Available for Sale Securities [Line Items]      
Available for sale securities, Amortized Cost 500 35,545  
Available for sale securities 472 35,430  
Available for sale securities, Gross Unrealized Gain 0 12  
Available for sale securities, Gross Unrealized Loss (28) (127)  
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] 7,982,265    
Available for Sale Securities, Maturity, Fair Value, Rolling Maturity [Abstract]      
Available for sale securities, Debt Maturities, without Single Maturity Date, Fair value $ 8,087,705    
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 $ 7,956,297 5,886,323  
Available for sale securities 8,046,096 5,804,708  
Available for sale securities, Gross Unrealized Gain 104,912 16,149  
Available for sale securities, Gross Unrealized Loss (15,113) (97,764)  
Residential non-agency mortgage-backed securities [Member]      
Available for Sale Securities [Line Items]      
Available for sale securities, Amortized Cost 25,968 40,948  
Available for sale securities 41,609 59,736  
Available for sale securities, Gross Unrealized Gain 15,641 18,788  
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 mortgage-backed securities were 4.0 years based upon current prepayment assumptions.