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Investment Securities (Tables) | 12 Months Ended
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Dec. 31, 2021
Investment Securities |
Schedule of amortized cost and fair value of securities | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | December 31,
​ | ​ | 2021 | ​ | 2020
​ | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | Fair | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | Fair
(dollars in thousands) |  | Cost |  | Gains |  | Losses |  | Value |  | Cost |  | Gains |  | Losses |  | Value
U.S. Treasury and government agency debt securities | ​ | $ | 196,662 | ​ | $ | 125 | ​ | $ | (4,224) | ​ | $ | 192,563 | ​ | $ | 170,123 | ​ | $ | 1,359 | ​ | $ | (61) | ​ | $ | 171,421
Mortgage-backed securities: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Residential - Government agency | ​ | ​ | 135,764 | ​ | ​ | 1,791 | ​ | ​ | (291) | ​ | ​ | 137,264 | ​ | ​ | 155,169 | ​ | ​ | 5,293 | ​ | ​ | — | ​ | ​ | 160,462
Residential - Government-sponsored enterprises | ​ | ​ | 1,496,605 | ​ | ​ | 6,914 | ​ | ​ | (12,419) | ​ | ​ | 1,491,100 | ​ | ​ | 434,282 | ​ | ​ | 13,643 | ​ | ​ | (725) | ​ | ​ | 447,200
Commercial - Government agency | ​ | ​ | 392,443 | ​ | ​ | 1,741 | ​ | ​ | (6,521) | ​ | ​ | 387,663 | ​ | ​ | 583,232 | ​ | ​ | 16,537 | ​ | ​ | (119) | ​ | ​ | 599,650
Commercial - Government-sponsored enterprises | ​ | ​ | 1,415,511 | ​ | ​ | 2,646 | ​ | ​ | (48,714) | ​ | ​ | 1,369,443 | ​ | ​ | 931,095 | ​ | ​ | 9,045 | ​ | ​ | (7,983) | ​ | ​ | 932,157
Collateralized mortgage obligations: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Government agency | ​ | ​ | 2,103,187 | ​ | ​ | 7,768 | ​ | ​ | (31,432) | ​ | ​ | 2,079,523 | ​ | ​ | 1,902,326 | ​ | ​ | 32,246 | ​ | ​ | (1,019) | ​ | ​ | 1,933,553
Government-sponsored enterprises | ​ | ​ | 2,671,131 | ​ | ​ | 3,608 | ​ | ​ | (53,695) | ​ | ​ | 2,621,044 | ​ | ​ | 1,808,804 | ​ | ​ | 18,991 | ​ | ​ | (823) | ​ | ​ | 1,826,972
Collateralized loan obligations | ​ | ​ | 105,245 | ​ | ​ | 2 | ​ | ​ | — | ​ | ​ | 105,247 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​ | ​ | —
Debt securities issued by states and political subdivisions | ​ | ​ | 44,185 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 44,185 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​ | ​ | —
Total available-for-sale securities | ​ | $ | 8,560,733 | ​ | $ | 24,595 | ​ | $ | (157,296) | ​ | $ | 8,428,032 | ​ | $ | 5,985,031 | ​ | $ | 97,114 | ​ | $ | (10,730) | ​ | $ | 6,071,415
Schedule of amortized cost and fair value of debt securities by contractual maturity | ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | December 31, 2021
​ | ​ | Amortized | ​ | Fair
(dollars in thousands) |  | Cost |  | Value
Due in one year or less | ​ | $ | — | ​ | $ | —
Due after one year through five years | ​ | ​ | 50,465 | ​ | ​ | 50,361
Due after five years through ten years | ​ | ​ | 123,729 | ​ | ​ | 121,960
Due after ten years | ​ | ​ | 171,898 | ​ | ​ | 169,674
​ | ​ | ​ | 346,092 | ​ | ​ | 341,995
​ | ​ | ​ | ​ | ​ | ​ | ​
Mortgage-backed securities: | ​ | ​ | ​ | ​ | ​ | ​
Residential - Government agency | ​ | ​ | 135,764 | ​ | ​ | 137,264
Residential - Government-sponsored enterprises | ​ | ​ | 1,496,605 | ​ | ​ | 1,491,100
Commercial - Government agency | ​ | ​ | 392,443 | ​ | ​ | 387,663
Commercial - Government-sponsored enterprises | ​ | ​ | 1,415,511 | ​ | ​ | 1,369,443
Total mortgage-backed securities | ​ | ​ | 3,440,323 | ​ | ​ | 3,385,470
Collateralized mortgage obligations: | ​ | ​ | ​ | ​ | ​ | ​
Government agency | ​ | ​ | 2,103,187 | ​ | ​ | 2,079,523
Government-sponsored enterprises | ​ | ​ | 2,671,131 | ​ | ​ | 2,621,044
Total collateralized mortgage obligations | ​ | ​ | 4,774,318 | ​ | ​ | 4,700,567
Total available-for-sale securities | ​ | $ | 8,560,733 | ​ | $ | 8,428,032
Schedule of gross unrealized losses and fair values of securities in a continuous loss position | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Time in Continuous Loss as of December 31, 2021
​ | ​ | Less Than 12 Months | ​ | 12 Months or More | ​ | Total
​ | ​ | Unrealized | ​ | ​ | ​ | ​ | Unrealized | ​ | ​ | ​ | ​ | Unrealized | ​ | ​ | ​
(dollars in thousands) |  | Losses |  | Fair Value |  | Losses |  | Fair Value |  | Losses |  | Fair Value
U.S. Treasury and government agency debt securities | ​ | $ | (3,355) | ​ | $ | 134,468 | ​ | $ | (869) | ​ | $ | 16,642 | ​ | $ | (4,224) | ​ | $ | 151,110
Mortgage-backed securities: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Residential - Government agency | ​ | ​ | (291) | ​ | ​ | 51,231 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | (291) | ​ | ​ | 51,231
Residential - Government-sponsored enterprises | ​ | ​ | (10,876) | ​ | ​ | 1,230,104 | ​ | ​ | (1,543) | ​ | ​ | 32,415 | ​ | ​ | (12,419) | ​ | ​ | 1,262,519
Commercial - Government agency | ​ | ​ | (5,239) | ​ | ​ | 186,024 | ​ | ​ | (1,282) | ​ | ​ | 26,063 | ​ | ​ | (6,521) | ​ | ​ | 212,087
Commercial - Government-sponsored enterprises | ​ | ​ | (22,179) | ​ | ​ | 744,819 | ​ | ​ | (26,535) | ​ | ​ | 397,123 | ​ | ​ | (48,714) | ​ | ​ | 1,141,942
Collateralized mortgage obligations: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Government agency | ​ | ​ | (31,432) | ​ | ​ | 1,441,848 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | (31,432) | ​ | ​ | 1,441,848
Government-sponsored enterprises | ​ | ​ | (52,551) | ​ | ​ | 2,255,535 | ​ | ​ | (1,144) | ​ | ​ | 24,959 | ​ | ​ | (53,695) | ​ | ​ | 2,280,494
Total available-for-sale securities with unrealized losses | ​ | $ | (125,923) | ​ | $ | 6,044,029 | ​ | $ | (31,373) | ​ | $ | 497,202 | ​ | $ | (157,296) | ​ | $ | 6,541,231
​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Time in Continuous Loss as of December 31, 2020
​ | ​ | Less Than 12 Months | ​ | 12 Months or More | ​ | Total
​ | ​ | Unrealized | ​ | ​ | ​ | ​ | Unrealized | ​ | ​ | ​ | ​ | Unrealized | ​ | ​ | ​
(dollars in thousands) |  | Losses |  | Fair Value |  | Losses |  | Fair Value |  | Losses |  | Fair Value
U.S. Treasury and government agency debt securities | ​ | $ | (61) | ​ | $ | 38,507 | ​ | $ | — | ​ | $ | — | ​ | $ | (61) | ​ | $ | 38,507
Mortgage-backed securities: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Residential - Government-sponsored enterprises | ​ | ​ | (725) | ​ | ​ | 64,987 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | (725) | ​ | ​ | 64,987
Commercial - Government agency | ​ | ​ | (119) | ​ | ​ | 32,346 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | (119) | ​ | ​ | 32,346
Commercial - Government-sponsored enterprises | ​ | ​ | (7,983) | ​ | ​ | 427,759 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | (7,983) | ​ | ​ | 427,759
Collateralized mortgage obligations: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Government agency | ​ | ​ | (994) | ​ | ​ | 209,124 | ​ | ​ | (25) | ​ | ​ | 6,190 | ​ | ​ | (1,019) | ​ | ​ | 215,314
Government-sponsored enterprises | ​ | ​ | (823) | ​ | ​ | 296,160 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | (823) | ​ | ​ | 296,160
Total available-for-sale securities with unrealized losses | ​ | $ | (10,705) | ​ | $ | 1,068,883 | ​ | $ | (25) | ​ | $ | 6,190 | ​ | $ | (10,730) | ​ | $ | 1,075,073

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