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Investment Securities Available for Sale (Amortized Cost and Estimated Fair Value by Contractual Maturity of Investment Securities Available for Sale) (Details) - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020
--- | --- | ---
Amortized Cost |  |
Within one year, amortized cost | $ 671,165 |
1 to 5 years, amortized cost | 120,002 |
5 to 10 years, amortized cost | 463,889 |
More than 10 years, amortized cost | 9,238,784 |
Amortized Cost | 10,493,840 | $ 7,801,864
Fair Value |  |
Within one year, fair value | 671,179 |
1 to 5 years, fair value | 123,758 |
5 to 10 years, fair value | 466,406 |
More than 10 years, fair value | 9,219,728 |
Total, fair value | 10,481,071 | 7,962,438
U.S. Treasury securities |  |
Amortized Cost |  |
Within one year, amortized cost | 20,350 |
1 to 5 years, amortized cost | 0 |
5 to 10 years, amortized cost | 99,862 |
More than 10 years, amortized cost | 0 |
Amortized Cost | 120,212 | 20,257
Fair Value |  |
Within one year, fair value | 20,350 |
1 to 5 years, fair value | 0 |
5 to 10 years, fair value | 97,543 |
More than 10 years, fair value | 0 |
Total, fair value | 117,893 | 20,257
U.S. Government agency securities |  |
Amortized Cost |  |
Within one year, amortized cost | 756 |
1 to 5 years, amortized cost | 321 |
5 to 10 years, amortized cost | 52,136 |
More than 10 years, amortized cost | 0 |
Amortized Cost | 53,213 | 79,638
Fair Value |  |
Within one year, fair value | 769 |
1 to 5 years, fair value | 326 |
5 to 10 years, fair value | 53,894 |
More than 10 years, fair value | 0 |
Total, fair value | 54,989 | 82,320
Mortgage-backed securities issued by U.S. Government agencies |  |
Amortized Cost |  |
Within one year, amortized cost | 0 |
1 to 5 years, amortized cost | 952 |
5 to 10 years, amortized cost | 140 |
More than 10 years, amortized cost | 878,096 |
Amortized Cost | 879,188 | 1,216,012
Fair Value |  |
Within one year, fair value | 0 |
1 to 5 years, fair value | 989 |
5 to 10 years, fair value | 146 |
More than 10 years, fair value | 871,082 |
Total, fair value | 872,217 | 1,218,017
Mortgage-backed securities issued by U.S. Government sponsored enterprises |  |
Amortized Cost |  |
Within one year, amortized cost | 59 |
1 to 5 years, amortized cost | 0 |
5 to 10 years, amortized cost | 71,559 |
More than 10 years, amortized cost | 7,208,953 |
Amortized Cost | 7,280,571 | 4,865,858
Fair Value |  |
Within one year, fair value | 60 |
1 to 5 years, fair value | 0 |
5 to 10 years, fair value | 74,540 |
More than 10 years, fair value | 7,196,333 |
Total, fair value | 7,270,933 | 5,000,046
Collateralized mortgage obligations issued by U.S. Government agencies or sponsored enterprises |  |
Amortized Cost |  |
Within one year, amortized cost | 0 |
1 to 5 years, amortized cost | 0 |
5 to 10 years, amortized cost | 165 |
More than 10 years, amortized cost | 1,060,638 |
Amortized Cost | 1,060,803 | 1,245,644
Fair Value |  |
Within one year, fair value | 0 |
1 to 5 years, fair value | 0 |
5 to 10 years, fair value | 172 |
More than 10 years, fair value | 1,057,316 |
Total, fair value | 1,057,488 | 1,250,377
Commercial mortgage-backed securities issued by U.S. Government agencies or sponsored enterprises |  |
Amortized Cost |  |
Within one year, amortized cost | 0 |
1 to 5 years, amortized cost | 109,227 |
5 to 10 years, amortized cost | 231,245 |
More than 10 years, amortized cost | 91,097 |
Amortized Cost | 431,569 | 354,244
Fair Value |  |
Within one year, fair value | 0 |
1 to 5 years, fair value | 112,628 |
5 to 10 years, fair value | 230,898 |
More than 10 years, fair value | 94,997 |
Total, fair value | 438,523 | 370,921
Asset-backed securities |  |
Amortized Cost |  |
Within one year, amortized cost | 650,000 |
1 to 5 years, amortized cost | 0 |
5 to 10 years, amortized cost | 0 |
More than 10 years, amortized cost | 0 |
Amortized Cost | 650,000 |
Fair Value |  |
Within one year, fair value | 650,000 |
1 to 5 years, fair value | 0 |
5 to 10 years, fair value | 0 |
More than 10 years, fair value | 0 |
Total, fair value | 650,000 |
Corporate debt securities and other debt securities |  |
Amortized Cost |  |
Within one year, amortized cost | 0 |
1 to 5 years, amortized cost | 9,502 |
5 to 10 years, amortized cost | 8,782 |
More than 10 years, amortized cost | 0 |
Amortized Cost | 18,284 | 20,211
Fair Value |  |
Within one year, fair value | 0 |
1 to 5 years, fair value | 9,815 |
5 to 10 years, fair value | 9,213 |
More than 10 years, fair value | 0 |
Total, fair value | $ 19,028 | $ 20,500

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