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Available for Sale Securities (Details)
$ in Thousands
3 Months Ended
Mar. 31, 2020
USD ($)
Mar. 31, 2019
USD ($)
Dec. 31, 2019
USD ($)
Debt Securities, Available-for-sale [Line Items]      
Available-for-sale Securities, Amortized Cost $ 12,258,288   $ 11,131,494
Available-for-sale Securities, Fair Value 12,694,277   11,269,643
Available-for-sale Securities, Gross Unrealized Gain 449,088   158,435
Available-for-sale Securities, Gross Unrealized Loss (13,099)   (20,286)
Debt Securities, Available-for-sale, Realized Gain (Loss) [Abstract]      
Available-for-sale Securities, Proceeds 26,894 $ 245,259  
Available-for-sale Securities, Gross realized gains 3 5,298  
Available-for-sale Securities, Gross realized losses 0 (5,222)  
Available-for-sale Securities, Related federal and state income tax expense (benefit) 1 $ 19  
Pledged Assets Separately Reported, Securities Pledged as Collateral, at Fair Value 13,100,000   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 34,239    
Available-for-sale Securities, Debt Maturities, One to Five Years, Amortized Cost Basis 1,223,377    
Available-for-sale Securities, Debt Maturities, Six To Ten Years, Amortized Cost Basis 1,400,140    
Available-for-sale Securities, Debt Maturities, Over Ten Years, Amortized Cost Basis 644,759    
Available-for-sale Securities, Debt Maturities, Allocated and Single Maturity Date, Amortized Cost 3,302,515    
Debt Securities, Available-for-sale, Maturity, Fair Value, Rolling Maturity [Abstract]      
Available-for-sale Securities, Debt Maturities, Less than One Year, Fair Value 34,320    
Available-for-sale Securities, Debt Maturities, One to Five Years, Fair Value 1,264,488    
Available-for-sale Securities, Debt Maturities, Six to Ten Years, Fair Value 1,439,807    
Available-for-sale Securities, Debt Maturities, Over Ten Years, Fair Value 661,707    
Available-for-sale Securities, Debt Maturities, Allocated and Single Maturity Date, Fair Value $ 3,400,322    
Available-for-sale Securities, Debt Maturities, Weighted Average Maturity [1] 8.34    
U.S. Treasury [Member]      
Debt Securities, Available-for-sale [Line Items]      
Available-for-sale Securities, Amortized Cost $ 901   1,598
Available-for-sale Securities, Fair Value 917   1,600
Available-for-sale Securities, Gross Unrealized Gain 16   2
Available-for-sale Securities, Gross Unrealized Loss 0   0
Municipal and other tax-exempt securities [Member]      
Debt Securities, Available-for-sale [Line Items]      
Available-for-sale Securities, Amortized Cost 23,166   1,789
Available-for-sale Securities, Fair Value 24,034   1,861
Available-for-sale Securities, Gross Unrealized Gain 872   72
Available-for-sale Securities, Gross Unrealized Loss (4)   0
Commercial agency mortgage-backed securities [Member]      
Debt Securities, Available-for-sale [Line Items]      
Available-for-sale Securities, Amortized Cost 3,277,948   3,145,342
Available-for-sale Securities, Fair Value 3,374,899   3,178,005
Available-for-sale Securities, Gross Unrealized Gain 103,953   37,808
Available-for-sale Securities, Gross Unrealized Loss (7,002)   (5,145)
Other debt securities [Member]      
Debt Securities, Available-for-sale [Line Items]      
Available-for-sale Securities, Amortized Cost 500   500
Available-for-sale Securities, Fair Value 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]      
Debt Securities, Available-for-sale, Maturity, Amortized Cost, Rolling Maturity [Abstract]      
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Amortized Cost [2] 8,955,773    
Debt Securities, Available-for-sale, Maturity, Fair Value, Rolling Maturity [Abstract]      
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Fair Value $ 9,293,955    
Available-for-sale Securities, Debt Maturities, Average Expected Life of Mortgage-backed Securities 3 years 3 months 18 days    
Residential agency mortgage-backed securities [Member]      
Debt Securities, Available-for-sale [Line Items]      
Available-for-sale Securities, Amortized Cost $ 8,931,771   7,956,297
Available-for-sale Securities, Fair Value 9,259,089   8,046,096
Available-for-sale Securities, Gross Unrealized Gain 333,298   104,912
Available-for-sale Securities, Gross Unrealized Loss (5,980)   (15,113)
Residential non-agency mortgage-backed securities [Member]      
Debt Securities, Available-for-sale [Line Items]      
Available-for-sale Securities, Amortized Cost 24,002   25,968
Available-for-sale Securities, Fair Value 34,866   41,609
Available-for-sale Securities, Gross Unrealized Gain 10,949   15,641
Available-for-sale Securities, Gross Unrealized Loss $ (85)   $ 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 3.3 years based upon current prepayment assumptions.