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Available for Sale Securities (Details)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2020
USD ($)
Sep. 30, 2019
USD ($)
Sep. 30, 2020
USD ($)
Sep. 30, 2019
USD ($)
Dec. 31, 2019
USD ($)
Debt Securities, Available-for-sale [Line Items]          
Available-for-sale Securities, Amortized Cost $ 12,336,706   $ 12,336,706   $ 11,131,494
Available-for-sale Securities, Fair Value 12,817,269   12,817,269   11,269,643
Available-for-sale Securities, Gross Unrealized Gain 485,489   485,489   158,435
Available-for-sale Securities, Gross Unrealized Loss (4,926)   (4,926)   (20,286)
Debt Securities, Available-for-sale, Realized Gain (Loss) [Abstract]          
Available-for-sale Securities, Proceeds 1,034 $ 261,028 206,979 $ 628,385  
Available-for-sale Securities, Gross realized gains 54 989 5,637 7,316  
Available-for-sale Securities, Gross realized losses (66) (984) (66) (6,206)  
Available-for-sale Securities, Related federal and state income tax expense (benefit) (3) $ 1 1,419 $ 282  
Pledged Assets Separately Reported, Securities Pledged as Collateral, at Fair Value 12,500,000   12,500,000   10,100,000
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 70,791   70,791    
Available-for-sale Securities, Debt Maturities, One to Five Years, Amortized Cost Basis 1,326,715   1,326,715    
Available-for-sale Securities, Debt Maturities, Six To Ten Years, Amortized Cost Basis 1,282,122   1,282,122    
Available-for-sale Securities, Debt Maturities, Over Ten Years, Amortized Cost Basis 603,396   603,396    
Available-for-sale Securities, Debt Maturities, Allocated and Single Maturity Date, Amortized Cost 3,283,024   3,283,024    
Debt Securities, Available-for-sale, Maturity, Fair Value, Rolling Maturity [Abstract]          
Available-for-sale Securities, Debt Maturities, Less than One Year, Fair Value 71,123   71,123    
Available-for-sale Securities, Debt Maturities, One to Five Years, Fair Value 1,383,695   1,383,695    
Available-for-sale Securities, Debt Maturities, Six to Ten Years, Fair Value 1,313,942   1,313,942    
Available-for-sale Securities, Debt Maturities, Over Ten Years, Fair Value 628,638   628,638    
Available-for-sale Securities, Debt Maturities, Allocated and Single Maturity Date, Fair Value $ 3,397,398   $ 3,397,398    
Available-for-sale Securities, Debt Maturities, Weighted Average Maturity [1] 8.02   8.02    
U.S. Treasury [Member]          
Debt Securities, Available-for-sale [Line Items]          
Available-for-sale Securities, Amortized Cost $ 500   $ 500   1,598
Available-for-sale Securities, Fair Value 509   509   1,600
Available-for-sale Securities, Gross Unrealized Gain 9   9   2
Available-for-sale Securities, Gross Unrealized Loss 0   0   0
Municipal and other tax-exempt securities [Member]          
Debt Securities, Available-for-sale [Line Items]          
Available-for-sale Securities, Amortized Cost 50,050   50,050   1,789
Available-for-sale Securities, Fair Value 51,601   51,601   1,861
Available-for-sale Securities, Gross Unrealized Gain 1,648   1,648   72
Available-for-sale Securities, Gross Unrealized Loss (97)   (97)   0
Commercial agency mortgage-backed securities [Member]          
Debt Securities, Available-for-sale [Line Items]          
Available-for-sale Securities, Amortized Cost 3,221,538   3,221,538   3,145,342
Available-for-sale Securities, Fair Value 3,334,409   3,334,409   3,178,005
Available-for-sale Securities, Gross Unrealized Gain 114,244   114,244   37,808
Available-for-sale Securities, Gross Unrealized Loss (1,373)   (1,373)   (5,145)
Other debt securities [Member]          
Debt Securities, Available-for-sale [Line Items]          
Available-for-sale Securities, Amortized Cost 10,936   10,936   500
Available-for-sale Securities, Fair Value 10,879   10,879   472
Available-for-sale Securities, Gross Unrealized Gain 0   0   0
Available-for-sale Securities, Gross Unrealized Loss (57)   (57)   (28)
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] 9,053,682   9,053,682    
Debt Securities, Available-for-sale, Maturity, Fair Value, Rolling Maturity [Abstract]          
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Fair Value 9,419,871   $ 9,419,871    
Available-for-sale Securities, Debt Maturities, Average Expected Life of Mortgage-backed Securities     3.02    
Residential agency mortgage-backed securities [Member]          
Debt Securities, Available-for-sale [Line Items]          
Available-for-sale Securities, Amortized Cost 9,033,728   $ 9,033,728   7,956,297
Available-for-sale Securities, Fair Value 9,384,998   9,384,998   8,046,096
Available-for-sale Securities, Gross Unrealized Gain 354,669   354,669   104,912
Available-for-sale Securities, Gross Unrealized Loss (3,399)   (3,399)   (15,113)
Residential non-agency mortgage-backed securities [Member]          
Debt Securities, Available-for-sale [Line Items]          
Available-for-sale Securities, Amortized Cost 19,954   19,954   25,968
Available-for-sale Securities, Fair Value 34,873   34,873   41,609
Available-for-sale Securities, Gross Unrealized Gain 14,919   14,919   15,641
Available-for-sale Securities, Gross Unrealized Loss $ 0   $ 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 3.02 years based upon current prepayment assumptions.