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Available for Sale Securities (Details)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2021
USD ($)
Sep. 30, 2020
USD ($)
Sep. 30, 2021
USD ($)
Sep. 30, 2020
USD ($)
Dec. 31, 2020
USD ($)
Debt Securities, Available-for-sale [Line Items]          
Available-for-sale Securities, Amortized Cost $ 13,120,626   $ 13,120,626   $ 12,609,850
Available-for-sale Securities, Fair Value 13,342,113   13,342,113   13,050,665
Available-for-sale Securities, Gross Unrealized Gain 292,863   292,863   449,654
Available-for-sale Securities, Gross Unrealized Loss (71,376)   (71,376)   (8,839)
Debt Securities, Available-for-sale, Realized Gain (Loss) [Abstract]          
Available-for-sale Securities, Proceeds 94,128 $ 1,034 488,274 $ 206,979  
Available-for-sale Securities, Gross realized gains 1,572 54 3,812 5,637  
Available-for-sale Securities, Gross realized losses (317) (66) (660) (66)  
Available-for-sale Securities, Related federal and state income tax expense (benefit) 301 $ (3) 756 $ 1,419  
Pledged Assets Separately Reported, Securities Pledged as Collateral, at Fair Value 8,800,000   8,800,000   11,600,000
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 62,357   62,357    
Available-for-sale Securities, Debt Maturities, One to Five Years, Amortized Cost Basis 2,015,024   2,015,024    
Available-for-sale Securities, Debt Maturities, Six To Ten Years, Amortized Cost Basis 2,495,766   2,495,766    
Available-for-sale Securities, Debt Maturities, Over Ten Years, Amortized Cost Basis 546,770   546,770    
Available-for-sale Securities, Debt Maturities, Allocated and Single Maturity Date, Amortized Cost 5,119,917   5,119,917    
Debt Securities, Available-for-sale, Maturity, Fair Value, Rolling Maturity [Abstract]          
Available-for-sale Securities, Debt Maturities, Less than One Year, Fair Value 62,773   62,773    
Available-for-sale Securities, Debt Maturities, One to Five Years, Fair Value 2,055,544   2,055,544    
Available-for-sale Securities, Debt Maturities, Six to Ten Years, Fair Value 2,475,406   2,475,406    
Available-for-sale Securities, Debt Maturities, Over Ten Years, Fair Value 554,293   554,293    
Available-for-sale Securities, Debt Maturities, Allocated and Single Maturity Date, Fair Value $ 5,148,016   $ 5,148,016    
Available-for-sale Securities, Debt Maturities, Weighted Average Maturity [1] 6.89   6.89    
U.S. Treasury [Member]          
Debt Securities, Available-for-sale [Line Items]          
Available-for-sale Securities, Amortized Cost $ 500   $ 500   500
Available-for-sale Securities, Fair Value 502   502   508
Available-for-sale Securities, Gross Unrealized Gain 2   2   8
Available-for-sale Securities, Gross Unrealized Loss 0   0   0
Municipal securities [Member]          
Debt Securities, Available-for-sale [Line Items]          
Available-for-sale Securities, Amortized Cost 464,368   464,368   165,318
Available-for-sale Securities, Fair Value 461,619   461,619   167,979
Available-for-sale Securities, Gross Unrealized Gain 1,396   1,396   2,666
Available-for-sale Securities, Gross Unrealized Loss (4,145)   (4,145)   (5)
Commercial agency mortgage-backed securities [Member]          
Debt Securities, Available-for-sale [Line Items]          
Available-for-sale Securities, Amortized Cost 4,654,549   4,654,549   3,406,956
Available-for-sale Securities, Fair Value 4,685,423   4,685,423   3,508,465
Available-for-sale Securities, Gross Unrealized Gain 60,223   60,223   103,590
Available-for-sale Securities, Gross Unrealized Loss (29,349)   (29,349)   (2,081)
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 472   472   472
Available-for-sale Securities, Gross Unrealized Gain 0   0   0
Available-for-sale Securities, Gross Unrealized Loss (28)   (28)   (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] 8,000,709   8,000,709    
Debt Securities, Available-for-sale, Maturity, Fair Value, Rolling Maturity [Abstract]          
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Fair Value 8,194,097   $ 8,194,097    
Available-for-sale Securities, Debt Maturities, Average Expected Life of Mortgage-backed Securities     3 years 7 months 6 days    
Residential agency mortgage-backed securities [Member]          
Debt Securities, Available-for-sale [Line Items]          
Available-for-sale Securities, Amortized Cost 7,988,834   $ 7,988,834   9,019,013
Available-for-sale Securities, Fair Value 8,168,025   8,168,025   9,340,471
Available-for-sale Securities, Gross Unrealized Gain 217,045   217,045   328,183
Available-for-sale Securities, Gross Unrealized Loss (37,854)   (37,854)   (6,725)
Residential non-agency mortgage-backed securities [Member]          
Debt Securities, Available-for-sale [Line Items]          
Available-for-sale Securities, Amortized Cost 11,875   11,875   17,563
Available-for-sale Securities, Fair Value 26,072   26,072   32,770
Available-for-sale Securities, Gross Unrealized Gain 14,197   14,197   15,207
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 3.6 years based upon current prepayment assumptions.