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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 | Jun. 30, 2020 | Dec. 31, 2019
--- | --- | ---
Amortized Cost |  |
Within One year, Amortized Cost | $ 44,538 |
1 to 5 Years, Amortized Cost | 224,144 |
5 to 10 Years, Amortized Cost | 350,039 |
More Than 10 years, Amortized Cost | 6,379,634 |
Amortized Cost | 6,998,355 | $ 6,647,791
Fair Value |  |
Within One Year, Fair Value | 44,459 |
1 to 5 Years, Fair Value | 230,196 |
5 to 10 Years, Fair Value | 367,877 |
More Than 10 years, Fair Value | 6,554,961 |
Total, Fair Value | 7,197,493 | 6,778,670
U.S. Treasury securities |  |
Amortized Cost |  |
Within One year, Amortized Cost | 19,921 |
1 to 5 Years, Amortized Cost | 0 |
5 to 10 Years, Amortized Cost | 0 |
More Than 10 years, Amortized Cost | 0 |
Amortized Cost | 19,921 | 19,855
Fair Value |  |
Within One Year, Fair Value | 19,921 |
1 to 5 Years, Fair Value | 0 |
5 to 10 Years, Fair Value | 0 |
More Than 10 years, Fair Value | 0 |
Total, Fair Value | 19,921 |
U.S. Government agency securities |  |
Amortized Cost |  |
Within One year, Amortized Cost | 568 |
1 to 5 Years, Amortized Cost | 27,714 |
5 to 10 Years, Amortized Cost | 140,636 |
More Than 10 years, Amortized Cost | 0 |
Amortized Cost | 168,918 | 35,499
Fair Value |  |
Within One Year, Fair Value | 571 |
1 to 5 Years, Fair Value | 27,770 |
5 to 10 Years, Fair Value | 143,025 |
More Than 10 years, Fair Value | 0 |
Total, Fair Value | 171,366 |
Mortgage-backed securities issued by U.S. Government agencies |  |
Amortized Cost |  |
Within One year, Amortized Cost | 0 |
1 to 5 Years, Amortized Cost | 1,398 |
5 to 10 Years, Amortized Cost | 715 |
More Than 10 years, Amortized Cost | 858,660 |
Amortized Cost | 860,773 | 56,328
Fair Value |  |
Within One Year, Fair Value | 0 |
1 to 5 Years, Fair Value | 1,442 |
5 to 10 Years, Fair Value | 743 |
More Than 10 years, Fair Value | 862,243 |
Total, Fair Value | 864,428 |
Mortgage-backed securities issued by U.S. Government sponsored enterprises |  |
Amortized Cost |  |
Within One year, Amortized Cost | 0 |
1 to 5 Years, Amortized Cost | 410 |
5 to 10 Years, Amortized Cost | 83,168 |
More Than 10 years, Amortized Cost | 4,209,985 |
Amortized Cost | 4,293,563 | 5,079,396
Fair Value |  |
Within One Year, Fair Value | 0 |
1 to 5 Years, Fair Value | 423 |
5 to 10 Years, Fair Value | 89,415 |
More Than 10 years, Fair Value | 4,354,414 |
Total, Fair Value | 4,444,252 |
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 | 273 |
More Than 10 years, Amortized Cost | 1,191,357 |
Amortized Cost | 1,191,630 |
Fair Value |  |
Within One Year, Fair Value | 0 |
1 to 5 Years, Fair Value | 0 |
5 to 10 Years, Fair Value | 285 |
More Than 10 years, Fair Value | 1,212,796 |
Total, Fair Value | 1,213,081 |
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 | 109,753 |
5 to 10 Years, Amortized Cost | 116,092 |
More Than 10 years, Amortized Cost | 117,129 |
Amortized Cost | 342,974 | 357,291
Fair Value |  |
Within One Year, Fair Value | 0 |
1 to 5 Years, Fair Value | 116,639 |
5 to 10 Years, Fair Value | 125,274 |
More Than 10 years, Fair Value | 123,340 |
Total, Fair Value | 365,253 | 371,592
State and municipal securities |  |
Amortized Cost |  |
Within One year, Amortized Cost | 0 |
1 to 5 Years, Amortized Cost | 0 |
5 to 10 Years, Amortized Cost | 500 |
More Than 10 years, Amortized Cost | 503 |
Amortized Cost | 1,003 | 2,069
Fair Value |  |
Within One Year, Fair Value | 0 |
1 to 5 Years, Fair Value | 0 |
5 to 10 Years, Fair Value | 501 |
More Than 10 years, Fair Value | 506 |
Total, Fair Value | 1,007 | 2,075
Corporate debt securities and other debt securities |  |
Amortized Cost |  |
Within One year, Amortized Cost | 24,049 |
1 to 5 Years, Amortized Cost | 84,869 |
5 to 10 Years, Amortized Cost | 8,655 |
More Than 10 years, Amortized Cost | 2,000 |
Amortized Cost | 119,573 | 144,410
Fair Value |  |
Within One Year, Fair Value | 23,967 |
1 to 5 Years, Fair Value | 83,922 |
5 to 10 Years, Fair Value | 8,634 |
More Than 10 years, Fair Value | 1,662 |
Total, Fair Value | $ 118,185 | $ 146,725

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