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Securities - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020
--- | --- | ---
Amortized cost and fair value of securities available for sale |  |
Amortized Cost | $ 2,532,767 | $ 1,301,973
Gross Unrealized Gains | 26,502 | 44,699
Gross Unrealized Losses | (14,626) | (3,215)
Fair Value | 2,544,643 | 1,343,457
U.S. Treasury securities |  |
Amortized cost and fair value of securities available for sale |  |
Amortized Cost | 3,014 | 7,047
Gross Unrealized Gains | 10 | 32
Gross Unrealized Losses | 0 | 0
Fair Value | 3,024 | 7,079
Obligations of other U.S. Government agencies and corporations |  |
Amortized cost and fair value of securities available for sale |  |
Amortized Cost |  | 1,003
Gross Unrealized Gains |  | 6
Gross Unrealized Losses |  | 0
Fair Value |  | 1,009
Obligations of states and political subdivisions |  |
Amortized cost and fair value of securities available for sale |  |
Amortized Cost | 372,311 | 291,231
Gross Unrealized Gains | 9,096 | 14,015
Gross Unrealized Losses | (2,426) | (45)
Fair Value | 378,981 | 305,201
Trust preferred securities |  |
Amortized cost and fair value of securities available for sale |  |
Amortized Cost |  | 12,013
Gross Unrealized Gains |  | 0
Gross Unrealized Losses |  | (3,001)
Fair Value |  | 9,012
Other debt securities |  |
Amortized cost and fair value of securities available for sale |  |
Amortized Cost | 51,321 | 62,771
Gross Unrealized Gains | 2,646 | 2,909
Gross Unrealized Losses | 0 | (73)
Fair Value | 53,967 | 65,607
Government agency mortgage backed securities | Residential mortgage backed securities: |  |
Amortized cost and fair value of securities available for sale |  |
Amortized Cost | 935,233 | 581,105
Gross Unrealized Gains | 11,694 | 21,564
Gross Unrealized Losses | (3,689) | (23)
Fair Value | 943,238 | 602,646
Government agency mortgage backed securities | Commercial mortgage backed securities: |  |
Amortized cost and fair value of securities available for sale |  |
Amortized Cost | 16,650 | 29,053
Gross Unrealized Gains | 515 | 1,235
Gross Unrealized Losses | (1) | (1)
Fair Value | 17,164 | 30,287
Government agency collateralized mortgage obligations | Residential mortgage backed securities: |  |
Amortized cost and fair value of securities available for sale |  |
Amortized Cost | 962,684 | 218,373
Gross Unrealized Gains | 1,225 | 1,946
Gross Unrealized Losses | (6,172) | (51)
Fair Value | 957,737 | 220,268
Government agency collateralized mortgage obligations | Commercial mortgage backed securities: |  |
Amortized cost and fair value of securities available for sale |  |
Amortized Cost | 191,554 | 99,377
Gross Unrealized Gains | 1,316 | 2,992
Gross Unrealized Losses | (2,338) | (21)
Fair Value | $ 190,532 | $ 102,348

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