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Securities - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands | Mar. 31, 2021 | Dec. 31, 2020
--- | --- | ---
Amortized cost and fair value of securities available for sale |  |
Amortized Cost | $ 1,515,958 | $ 1,301,973
Gross Unrealized Gains | 30,566 | 44,699
Gross Unrealized Losses | (10,483) | (3,215)
Fair Value | 1,536,041 | 1,343,457
U.S. Treasury securities |  |
Amortized cost and fair value of securities available for sale |  |
Amortized Cost | 3,028 | 7,047
Gross Unrealized Gains | 22 | 32
Gross Unrealized Losses | 0 | 0
Fair Value | 3,050 | 7,079
Obligations of other U.S. Government agencies and corporations |  |
Amortized cost and fair value of securities available for sale |  |
Amortized Cost | 1,001 | 1,003
Gross Unrealized Gains | 3 | 6
Gross Unrealized Losses | 0 | 0
Fair Value | 1,004 | 1,009
Obligations of states and political subdivisions |  |
Amortized cost and fair value of securities available for sale |  |
Amortized Cost | 321,156 | 291,231
Gross Unrealized Gains | 9,344 | 14,015
Gross Unrealized Losses | (1,682) | (45)
Fair Value | 328,818 | 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 | 61,113 | 62,771
Gross Unrealized Gains | 2,126 | 2,909
Gross Unrealized Losses | (40) | (73)
Fair Value | 63,199 | 65,607
Government agency mortgage backed securities | Residential mortgage backed securities: |  |
Amortized cost and fair value of securities available for sale |  |
Amortized Cost | 453,792 | 581,105
Gross Unrealized Gains | 14,869 | 21,564
Gross Unrealized Losses | (2,206) | (23)
Fair Value | 466,455 | 602,646
Government agency mortgage backed securities | Commercial mortgage backed securities: |  |
Amortized cost and fair value of securities available for sale |  |
Amortized Cost | 22,448 | 29,053
Gross Unrealized Gains | 580 | 1,235
Gross Unrealized Losses | (23) | (1)
Fair Value | 23,005 | 30,287
Government agency collateralized mortgage obligations | Residential mortgage backed securities: |  |
Amortized cost and fair value of securities available for sale |  |
Amortized Cost | 538,317 | 218,373
Gross Unrealized Gains | 1,733 | 1,946
Gross Unrealized Losses | (4,718) | (51)
Fair Value | 535,332 | 220,268
Government agency collateralized mortgage obligations | Commercial mortgage backed securities: |  |
Amortized cost and fair value of securities available for sale |  |
Amortized Cost | 115,103 | 99,377
Gross Unrealized Gains | 1,889 | 2,992
Gross Unrealized Losses | (1,814) | (21)
Fair Value | $ 115,178 | $ 102,348

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