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Investment Securities Available for Sale (Tables) | 12 Months Ended
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Dec. 31, 2022
Investments [Abstract] |
Summary of Available-for-Sale Investment Securities | The amortized cost, gross unrealized gains and losses, and estimated fair values of investment securities available for sale at December 31, 2022 and 2021 are summarized below.
|  | December 31, 2022
(in thousands) |  | Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Fair Value
U.S. Treasury securities |  | $ | 515,953 |  |  | $ | — |  |  | $ | (44,140) |  |  | $ | 471,813 |
U.S. Government agency securities |  | 52,411 |  |  | — |  |  | (3,613) |  |  | 48,798 |
Mortgage-backed securities issued by U.S. Government agencies |  | 904,593 |  |  | 1,624 |  |  | (113,468) |  |  | 792,749 |
Mortgage-backed securities issued by U.S. Government sponsored enterprises |  | 8,144,374 |  |  | 936 |  |  | (1,250,240) |  |  | 6,895,070 |
Collateralized mortgage obligations issued by U.S. Government agencies or sponsored enterprises |  | 769,498 |  |  | — |  |  | (114,371) |  |  | 655,127 |
Commercial mortgage-backed securities issued by U.S. Government agencies or sponsored enterprises |  | 877,590 |  |  | — |  |  | (71,645) |  |  | 805,945 |
Corporate debt securities and other debt securities |  | 8,908 |  |  | — |  |  | (307) |  |  | 8,601 |
Total investment securities available for sale |  | $ | 11,273,327 |  |  | $ | 2,560 |  |  | $ | (1,597,784) |  |  | $ | 9,678,103 |
|  | December 31, 2021
(in thousands) |  | Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Fair Value
U.S. Treasury securities |  | $ | 120,465 |  |  | $ | — |  |  | $ | (2,627) |  |  | $ | 117,838 |
U.S. Government agency securities |  | 53,214 |  |  | 1,374 |  |  | (387) |  |  | 54,201 |
Mortgage-backed securities issued by U.S. Government agencies |  | 790,329 |  |  | 768 |  |  | (11,464) |  |  | 779,633 |
Mortgage-backed securities issued by U.S. Government sponsored enterprises |  | 8,063,890 |  |  | 50,491 |  |  | (102,080) |  |  | 8,012,301 |
Collateralized mortgage obligations issued by U.S. Government agencies or sponsored enterprises |  | 951,691 |  |  | 4,658 |  |  | (16,726) |  |  | 939,623 |
Commercial mortgage-backed securities issued by U.S. Government agencies or sponsored enterprises |  | 479,420 |  |  | 8,644 |  |  | (6,320) |  |  | 481,744 |
Asset-backed securities |  | 514,188 |  |  | — |  |  | — |  |  | 514,188 |
Corporate debt securities and other debt securities |  | 18,309 |  |  | 492 |  |  | — |  |  | 18,801 |
Total investment securities available for sale |  | $ | 10,991,506 |  |  | $ | 66,427 |  |  | $ | (139,604) |  |  | $ | 10,918,329 |
Schedule of Unrealized Loss on Investments | Gross unrealized losses on investment securities and the fair value of the related securities, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position, at December 31, 2022 and December 31, 2021 are presented below.
|  | December 31, 2022
|  | Less than 12 Months |  | 12 Months or Longer |  | Total
(in thousands) |  | Fair Value |  | Gross Unrealized Losses |  | Fair Value |  | Gross Unrealized Losses |  | Fair Value |  | Gross Unrealized Losses
U.S. Treasury securities |  | $ | 139,737 |  |  | $ | (6,789) |  |  | $ | 307,582 |  |  | $ | (37,351) |  |  | $ | 447,319 |  |  | $ | (44,140) |
U.S. Government agency securities |  | 28,938 |  |  | (1,053) |  |  | 19,603 |  |  | (2,560) |  |  | 48,541 |  |  | (3,613) |
Mortgage-backed securities issued by U.S. Government agencies |  | 187,655 |  |  | (5,952) |  |  | 521,395 |  |  | (107,516) |  |  | 709,050 |  |  | (113,468) |
Mortgage-backed securities issued by U.S. Government sponsored enterprises |  | 1,473,348 |  |  | (120,135) |  |  | 5,365,233 |  |  | (1,130,105) |  |  | 6,838,581 |  |  | (1,250,240) |
Collateralized mortgage obligations issued by U.S. Government agencies or sponsored enterprises |  | 119,649 |  |  | (10,311) |  |  | 535,478 |  |  | (104,060) |  |  | 655,127 |  |  | (114,371) |
Commercial mortgage-backed securities issued by U.S. Government agencies or sponsored enterprises |  | 565,382 |  |  | (29,383) |  |  | 240,564 |  |  | (42,262) |  |  | 805,946 |  |  | (71,645) |
Corporate debt securities and other debt securities |  | 8,601 |  |  | (307) |  |  | — |  |  | — |  |  | 8,601 |  |  | (307) |
Total |  | $ | 2,523,310 |  |  | $ | (173,930) |  |  | $ | 6,989,855 |  |  | $ | (1,423,854) |  |  | $ | 9,513,165 |  |  | $ | (1,597,784) |
|  | December 31, 2021
|  | Less than 12 Months |  | 12 Months or Longer |  | Total
(in thousands) |  | Fair Value |  | Gross Unrealized Losses |  | Fair Value |  | Gross Unrealized Losses |  | Fair Value |  | Gross Unrealized Losses
U.S. Treasury securities |  | $ | 49,648 |  |  | $ | (379) |  |  | $ | 47,590 |  |  | $ | (2,248) |  |  | $ | 97,238 |  |  | $ | (2,627) |
U.S. Government agency securities |  | 21,760 |  |  | (387) |  |  | — |  |  | — |  |  | 21,760 |  |  | (387) |
Mortgage-backed securities issued by U.S. Government agencies |  | 461,078 |  |  | (5,858) |  |  | 244,264 |  |  | (5,606) |  |  | 705,342 |  |  | (11,464) |
Mortgage-backed securities issued by U.S. Government sponsored enterprises |  | 5,729,476 |  |  | (82,671) |  |  | 643,758 |  |  | (19,409) |  |  | 6,373,234 |  |  | (102,080) |
Collateralized mortgage obligations issued by U.S. Government agencies or sponsored enterprises |  | 187,431 |  |  | (3,981) |  |  | 504,238 |  |  | (12,745) |  |  | 691,669 |  |  | (16,726) |
Commercial mortgage-backed securities issued by U.S. Government agencies or sponsored enterprises |  | 146,672 |  |  | (2,951) |  |  | 83,533 |  |  | (3,369) |  |  | 230,205 |  |  | (6,320) |
Total |  | $ | 6,596,065 |  |  | $ | (96,227) |  |  | $ | 1,523,383 |  |  | $ | (43,377) |  |  | $ | 8,119,448 |  |  | $ | (139,604) |
Amortized Cost and Estimated Fair Value by Contractual Maturity of Investment Securities Available-for-Sale | The amortized cost and fair value by contractual maturity of investment securities available for sale at December 31, 2022 are shown below. The expected life of MBSs or CMOs may differ from contractual maturities because issuers may have the right to call or prepay obligations with or without call or prepayment penalties. For purposes of the maturity table, MBSs and CMOs, which are not due at a single maturity date, have been classified based on the final contractual maturity date.
|  | Distribution of Maturities at December 31, 2022
(in thousands) |  | Within One Year |  | 1 to 5 Years |  | 5 to 10 Years |  | More Than 10 Years |  | Total
Amortized Cost |  |  |  |  |  |  |  |  |  |
U.S. Treasury securities |  | $ | 24,293 |  |  | $ | 441,804 |  |  | $ | 49,856 |  |  | $ | — |  |  | $ | 515,953 |
U.S. Government agency securities |  | 273 |  |  | 22,148 |  |  | 29,990 |  |  | — |  |  | 52,411 |
Mortgage-backed securities issued by U.S. Government agencies |  | — |  |  | 534 |  |  | 4 |  |  | 904,055 |  |  | 904,593 |
Mortgage-backed securities issued by U.S. Government sponsored enterprises |  | — |  |  | 4,659 |  |  | 109,583 |  |  | 8,030,132 |  |  | 8,144,374 |
Collateralized mortgage obligations issued by U.S. Government agencies or sponsored enterprises |  | — |  |  | 91 |  |  | 12,293 |  |  | 757,114 |  |  | 769,498 |
Commercial mortgage-backed securities issued by U.S. Government agencies or sponsored enterprises |  | — |  |  | 573,334 |  |  | 238,228 |  |  | 66,028 |  |  | 877,590 |
Corporate debt securities and other debt securities |  | — |  |  | 8,908 |  |  | — |  |  | — |  |  | 8,908 |
Total amortized cost |  | $ | 24,566 |  |  | $ | 1,051,478 |  |  | $ | 439,954 |  |  | $ | 9,757,329 |  |  | $ | 11,273,327 |
Fair Value |  |  |  |  |  |  |  |  |  |
U.S. Treasury securities |  | $ | 24,293 |  |  | $ | 407,430 |  |  | $ | 40,090 |  |  | $ | — |  |  | $ | 471,813 |
U.S. Government agency securities |  | 273 |  |  | 19,588 |  |  | 28,937 |  |  | — |  |  | 48,798 |
Mortgage-backed securities issued by U.S. Government agencies |  | — |  |  | 517 |  |  | 4 |  |  | 792,228 |  |  | 792,749 |
Mortgage-backed securities issued by U.S. Government sponsored enterprises |  | — |  |  | 4,451 |  |  | 102,400 |  |  | 6,788,219 |  |  | 6,895,070 |
Collateralized mortgage obligations issued by U.S. Government agencies or sponsored enterprises |  | — |  |  | 88 |  |  | 11,808 |  |  | 643,231 |  |  | 655,127 |
Commercial mortgage-backed securities issued by U.S. Government agencies or sponsored enterprises |  | — |  |  | 540,820 |  |  | 203,757 |  |  | 61,368 |  |  | 805,945 |
Corporate debt securities and other debt securities |  | — |  |  | 8,601 |  |  | — |  |  | — |  |  | 8,601 |
Total fair value |  | $ | 24,566 |  |  | $ | 981,495 |  |  | $ | 386,996 |  |  | $ | 8,285,046 |  |  | $ | 9,678,103 |
Summary of Sales Transactions in the Investment Securities Available-for-Sale Portfolio | Gross gains and gross losses on sales of securities available for sale for the years ended December 31, 2022, 2021, and 2020 are presented below. The specific identification method is used to reclassify gains and losses out of other comprehensive income (loss) at the time of sale.
(in thousands) |  | 2022 |  | 2021 |  | 2020
Gross realized gains on sales |  | $ | — |  |  | $ | 1,191 |  |  | $ | 85,375 |
Gross realized losses on sales |  | — |  |  | (1,990) |  |  | (6,444) |
Investment securities gains (losses), net |  | $ | — |  |  | $ | (799) |  |  | $ | 78,931 |

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