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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, 2020 | Dec. 31, 2019
--- | --- | ---
Amortized Cost |  |
Within One year, Amortized Cost | $ 20,684 |
1 to 5 Years, Amortized Cost | 147,563 |
5 to 10 Years, Amortized Cost | 321,871 |
More Than 10 years, Amortized Cost | 6,905,147 |
Amortized Cost | 7,395,265 | $ 6,647,791
Fair Value |  |
Within One Year, Fair Value | 20,692 |
1 to 5 Years, Fair Value | 154,223 |
5 to 10 Years, Fair Value | 333,954 |
More Than 10 years, Fair Value | 7,057,656 |
Total, Fair Value | 7,566,525 | 6,778,670
U.S. Treasury securities |  |
Amortized Cost |  |
Within One year, Amortized Cost | 20,254 |
1 to 5 Years, Amortized Cost | 0 |
5 to 10 Years, Amortized Cost | 0 |
More Than 10 years, Amortized Cost | 0 |
Amortized Cost | 20,254 | 19,855
Fair Value |  |
Within One Year, Fair Value | 20,254 |
1 to 5 Years, Fair Value | 0 |
5 to 10 Years, Fair Value | 0 |
More Than 10 years, Fair Value | 0 |
Total, Fair Value | 20,254 | 19,855
U.S. Government agency securities |  |
Amortized Cost |  |
Within One year, Amortized Cost | 430 |
1 to 5 Years, Amortized Cost | 26,432 |
5 to 10 Years, Amortized Cost | 127,122 |
More Than 10 years, Amortized Cost | 0 |
Amortized Cost | 153,984 | 35,499
Fair Value |  |
Within One Year, Fair Value | 438 |
1 to 5 Years, Fair Value | 26,243 |
5 to 10 Years, Fair Value | 129,933 |
More Than 10 years, Fair Value | 0 |
Total, Fair Value | 156,614 | 36,541
Mortgage-backed securities issued by U.S. Government agencies |  |
Amortized Cost |  |
Within One year, Amortized Cost | 0 |
1 to 5 Years, Amortized Cost | 1,585 |
5 to 10 Years, Amortized Cost | 311 |
More Than 10 years, Amortized Cost | 1,130,920 |
Amortized Cost | 1,132,816 | 56,328
Fair Value |  |
Within One Year, Fair Value | 0 |
1 to 5 Years, Fair Value | 1,641 |
5 to 10 Years, Fair Value | 324 |
More Than 10 years, Fair Value | 1,127,427 |
Total, Fair Value | 1,129,392 | 56,816
Mortgage-backed securities issued by U.S. Government sponsored enterprises |  |
Amortized Cost |  |
Within One year, Amortized Cost | 0 |
1 to 5 Years, Amortized Cost | 310 |
5 to 10 Years, Amortized Cost | 73,420 |
More Than 10 years, Amortized Cost | 4,328,321 |
Amortized Cost | 4,402,051 | 5,079,396
Fair Value |  |
Within One Year, Fair Value | 0 |
1 to 5 Years, Fair Value | 319 |
5 to 10 Years, Fair Value | 76,148 |
More Than 10 years, Fair Value | 4,463,919 |
Total, Fair Value | 4,540,386 | 5,180,815
Collateralized Mortgage Backed Securities Issued By U S Government Sponsored Enterprises |  |
Amortized Cost |  |
Within One year, Amortized Cost | 0 |
1 to 5 Years, Amortized Cost | 0 |
5 to 10 Years, Amortized Cost | 248 |
More Than 10 years, Amortized Cost | 1,330,763 |
Amortized Cost | 1,331,011 |
Fair Value |  |
Within One Year, Fair Value | 0 |
1 to 5 Years, Fair Value | 0 |
5 to 10 Years, Fair Value | 259 |
More Than 10 years, Fair Value | 1,345,025 |
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 | 109,731 |
5 to 10 Years, Amortized Cost | 112,089 |
More Than 10 years, Amortized Cost | 112,643 |
Amortized Cost | 334,463 | 357,291
Fair Value |  |
Within One Year, Fair Value | 0 |
1 to 5 Years, Fair Value | 116,705 |
5 to 10 Years, Fair Value | 118,468 |
More Than 10 years, Fair Value | 118,984 |
Total, Fair Value | 354,157 | 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 | 0 |
More Than 10 years, Amortized Cost | 500 |
Amortized Cost | 500 | 2,069
Fair Value |  |
Within One Year, Fair Value | 0 |
1 to 5 Years, Fair Value | 0 |
5 to 10 Years, Fair Value | 0 |
More Than 10 years, Fair Value | 501 |
Total, Fair Value | 501 | 2,075
Corporate debt securities and other debt securities |  |
Amortized Cost |  |
Within One year, Amortized Cost | 0 |
1 to 5 Years, Amortized Cost | 9,505 |
5 to 10 Years, Amortized Cost | 8,681 |
More Than 10 years, Amortized Cost | 2,000 |
Amortized Cost | 20,186 | 144,410
Fair Value |  |
Within One Year, Fair Value | 0 |
1 to 5 Years, Fair Value | 9,315 |
5 to 10 Years, Fair Value | 8,822 |
More Than 10 years, Fair Value | 1,800 |
Total, Fair Value | 19,937 | 146,725
Collateralized mortgage obligations issued by U.S. Government agencies or sponsored enterprises |  |
Amortized Cost |  |
Amortized Cost | 1,331,011 | 629,706
Fair Value |  |
Total, Fair Value | $ 1,345,284 | $ 636,851

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