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Investment Securities and Equity Securities with Readily Determinable Fair Values (Details) - USD ($)
$ in Thousands
Mar. 31, 2021
Dec. 31, 2020
Held-to-maturity securities    
Amortized cost $ 3,400 $ 3,400
Estimated fair value 3,400 3,400
Carrying Value 3,400 3,400
Available-for-sale securities    
Amortized cost 4,034,746 3,054,289
Available for sale debt securities 4,057,357 3,080,768
Total Amortized cost 4,034,746 3,054,289
Total Gross unrealized gains 31,741 35,832
Total Gross unrealized losses 9,130 9,353
Estimate of Fair Value Measurement [Member]    
Available-for-sale securities    
Available for sale debt securities and equity securities 4,057,357 3,080,768
Reported Value Measurement [Member]    
Available-for-sale securities    
Available for sale debt securities and equity securities 4,057,357 3,080,768
US Treasury Securities [Member]    
Available-for-sale securities    
Amortized cost 945,682  
US Treasury Securities [Member] | Estimate of Fair Value Measurement [Member]    
Available-for-sale securities    
Available for sale debt securities 945,682  
US Treasury Securities [Member] | Reported Value Measurement [Member]    
Available-for-sale securities    
Available for sale debt securities 945,682  
Collateralized Mortgage Backed Securities [Member]    
Available-for-sale securities    
Amortized cost 3,043,953 3,006,592
Gross unrealized gains 28,203 32,701
Gross unrealized losses (9,120) (9,339)
Collateralized Mortgage Backed Securities [Member] | Estimate of Fair Value Measurement [Member]    
Available-for-sale securities    
Available for sale debt securities 3,063,036 3,029,954
Collateralized Mortgage Backed Securities [Member] | Reported Value Measurement [Member]    
Available-for-sale securities    
Available for sale debt securities 3,063,036 3,029,954
US Government Corporations and Agencies Securities [Member]    
Available-for-sale securities    
Available for sale debt securities 326,756 371,407
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]    
Available-for-sale securities    
Available for sale debt securities 2,736,280 2,658,547
US States and Political Subdivisions Debt Securities [Member]    
Available-for-sale securities    
Amortized cost 45,111 47,697
Gross unrealized gains 3,538 3,131
Gross unrealized losses (10) (14)
US States and Political Subdivisions Debt Securities [Member] | Estimate of Fair Value Measurement [Member]    
Available-for-sale securities    
Available for sale debt securities 48,639 50,814
US States and Political Subdivisions Debt Securities [Member] | Reported Value Measurement [Member]    
Available-for-sale securities    
Available for sale debt securities 48,639 50,814
Other Debt Obligations [Member]    
Held-to-maturity securities    
Amortized cost 3,400 3,400
Estimated fair value 3,400 3,400
Carrying Value $ 3,400 $ 3,400