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Investment Securities - Gross unrealized losses and fair value (Details)
$ in Thousands
Mar. 31, 2022
USD ($)
security
Dec. 31, 2021
USD ($)
security
Debt Securities, Available-for-sale [Line Items]    
Number of securities | security 229 239
Less than 12 months, estimated fair value $ 1,337,468 $ 1,891,310
Less than 12 months, unrealized losses 68,770 24,285
12 months or greater, estimated fair value 353,903 177,038
12 months or greater, unrealized losses 29,713 5,322
Estimated fair value 1,691,371 2,068,348
Unrealized losses $ 98,483 $ 29,607
U.S. treasury bonds    
Debt Securities, Available-for-sale [Line Items]    
Number of securities | security 2 1
Less than 12 months, estimated fair value $ 47,492 $ 24,593
Less than 12 months, unrealized losses 2,225 257
12 months or greater, estimated fair value 0 0
12 months or greater, unrealized losses 0 0
Estimated fair value 47,492 24,593
Unrealized losses $ 2,225 $ 257
U. S. agency securities    
Debt Securities, Available-for-sale [Line Items]    
Number of securities | security 80 64
Less than 12 months, estimated fair value $ 469,018 $ 452,966
Less than 12 months, unrealized losses 22,925 6,256
12 months or greater, estimated fair value 194,355 68,977
12 months or greater, unrealized losses 15,032 1,366
Estimated fair value 663,373 521,943
Unrealized losses $ 37,957 $ 7,622
Residential mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Number of securities | security 146 153
Less than 12 months, estimated fair value $ 812,079 $ 1,327,519
Less than 12 months, unrealized losses 43,414 16,841
12 months or greater, estimated fair value 159,548 108,061
12 months or greater, unrealized losses 14,681 3,956
Estimated fair value 971,627 1,435,580
Unrealized losses $ 58,095 $ 20,797
Municipal bonds    
Debt Securities, Available-for-sale [Line Items]    
Number of securities | security 1 8
Less than 12 months, estimated fair value $ 8,879 $ 20,181
Less than 12 months, unrealized losses 206 347
12 months or greater, estimated fair value 0 0
12 months or greater, unrealized losses 0 0
Estimated fair value 8,879 20,181
Unrealized losses $ 206 $ 347
Corporate bonds    
Debt Securities, Available-for-sale [Line Items]    
Number of securities | security   13
Less than 12 months, estimated fair value   $ 66,051
Less than 12 months, unrealized losses   584
12 months or greater, estimated fair value   0
12 months or greater, unrealized losses   0
Estimated fair value   66,051
Unrealized losses   $ 584