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Securities (Tables) | 9 Months Ended
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Sep. 30, 2017
Investments, Debt and Equity Securities [Abstract] |
Schedule of amortized cost, gross unrealized gains, gross unrealized losses and fair value by major categories of investment securities | The following tables present the amortized cost, gross unrealized gains and losses and fair value by major categories of available-for-sale investment securities, which are carried at fair value, and the held-to-maturity investment security, which is carried at amortized cost, as of September 30, 2017 and December 31, 2016:
|  | As of September 30, 2017
($ in thousands) |  | Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Fair Value
Available-for-sale investment securities: |  |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury securities |  | $ | 533,035 |  |  | $ | — |  |  | $ | (6,703 | ) |  | $ | 526,332 |
U.S. government agency and U.S. government sponsored enterprise debt securities |  | 191,727 |  |  | 81 |  |  | (2,623 | ) |  | 189,185 |
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities: |  |  |  |  |  |  |  |  |  |  |  |
Commercial mortgage-backed securities |  | 321,943 |  |  | 326 |  |  | (7,097 | ) |  | 315,172 |
Residential mortgage-backed securities |  | 1,154,026 |  |  | 4,790 |  |  | (7,882 | ) |  | 1,150,934 |
Municipal securities |  | 116,798 |  |  | 900 |  |  | (456 | ) |  | 117,242 |
Non-agency residential mortgage-backed securities: |  |  |  |  |  |  |  |  |
Investment grade (1) |  | 9,680 |  |  | 21 |  |  | (7 | ) |  | 9,694 |
Corporate debt securities: |  |  |  |  |  |  |  |  |
Investment grade (1) |  | 2,464 |  |  | — |  |  | (137 | ) |  | 2,327 |
Non-investment grade (1) |  | 10,191 |  |  | — |  |  | (576 | ) |  | 9,615 |
Foreign bonds: |  |  |  |  |  |  |  |  |
Investment grade (1) (2) |  | 505,395 |  |  | 229 |  |  | (16,484 | ) |  | 489,140 |
Other securities (3) |  | 147,504 |  |  | 3 |  |  | (372 | ) |  | 147,135 |
Total available-for-sale investment securities |  | $ | 2,992,763 |  |  | $ | 6,350 |  |  | $ | (42,337 | ) |  | $ | 2,956,776 |
Held-to-maturity investment security: |  |  |  |  |  |  |  |
Non-agency commercial mortgage-backed security |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | — |
Total investment securities |  | $ | 2,992,763 |  |  | $ | 6,350 |  |  | $ | (42,337 | ) |  | $ | 2,956,776 |
|  | As of December 31, 2016
($ in thousands) |  | Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Fair Value
Available-for-sale investment securities: |  |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury securities |  | $ | 730,287 |  |  | $ | 21 |  |  | $ | (9,829 | ) |  | $ | 720,479 |
U.S. government agency and U.S. government sponsored enterprise debt securities |  | 277,891 |  |  | 224 |  |  | (3,249 | ) |  | 274,866 |
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities: |  |  |  |  |  |  |  |  |  |  |  |
Commercial mortgage-backed securities |  | 272,672 |  |  | 345 |  |  | (6,218 | ) |  | 266,799 |
Residential mortgage-backed securities |  | 1,266,372 |  |  | 3,924 |  |  | (11,549 | ) |  | 1,258,747 |
Municipal securities |  | 148,302 |  |  | 1,252 |  |  | (1,900 | ) |  | 147,654 |
Non-agency residential mortgage-backed securities: |  |  |  |  |  |  |  |  |
Investment grade (1) |  | 11,592 |  |  | — |  |  | (115 | ) |  | 11,477 |
Corporate debt securities: |  |  |  |  |  |  |  |  |
Investment grade (1) |  | 222,190 |  |  | 562 |  |  | (375 | ) |  | 222,377 |
Non-investment grade (1) |  | 10,191 |  |  | — |  |  | (1,018 | ) |  | 9,173 |
Foreign bonds: |  |  |  |  |  |  |  |
Investment grade (1) (2) |  | 405,443 |  |  | 30 |  |  | (21,579 | ) |  | 383,894 |
Other securities |  | 40,501 |  |  | 337 |  |  | (509 | ) |  | 40,329 |
Total available-for-sale investment securities |  | $ | 3,385,441 |  |  | $ | 6,695 |  |  | $ | (56,341 | ) |  | $ | 3,335,795 |
Held-to-maturity investment security: |  |  |  |  |  |  |  |
Non-agency commercial mortgage-backed security (3) |  | $ | 143,971 |  |  | $ | 622 |  |  | $ | — |  |  | $ | 144,593 |
Total investment securities |  | $ | 3,529,412 |  |  | $ | 7,317 |  |  | $ | (56,341 | ) |  | $ | 3,480,388 |
(1) | Available-for-sale investment securities rated BBB- or higher by Standard &Poor’s (“S&P”) or Baa3 or higher by Moody’s are considered investment grade. Conversely, available-for-sale investment securities rated lower than BBB- by S&P or lower than Baa3 by Moody’s are considered non-investment grade. Classifications are based on the lower of the credit ratings by S&P or Moody’s.
(2) | Fair values of foreign bonds include $458.9 million and $353.6 million of multilateral development bank bonds as of September 30, 2017 and December 31, 2016, respectively.
(3) | During the third quarter of 2017, the Company transferred a non-agency commercial mortgage-backed security with a net carrying amount of $115.6 million from held-to-maturity to available-for-sale.
Schedule of gross unrealized losses and related fair value of investment securities | The following tables present the gross unrealized losses and related fair values of the Company’s investment portfolio, aggregated by investment category and the length of time that individual securities have been in a continuous unrealized loss position, as of September 30, 2017 and December 31, 2016:
|  | As of September 30, 2017
($ in thousands) |  | Less Than 12 Months |  | 12 Months or More |  | Total
| Fair Value |  | Gross Unrealized Losses |  | Fair Value |  | Gross Unrealized Losses |  | Fair Value |  | Gross Unrealized Losses
Available-for-sale investment securities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury securities |  | $ | 415,507 |  |  | $ | (4,615 | ) |  | $ | 110,825 |  |  | $ | (2,088 | ) |  | $ | 526,332 |  |  | $ | (6,703 | )
U.S. government agency and U.S. government sponsored enterprise debt securities |  | 96,681 |  |  | (367 | ) |  | 54,512 |  |  | (2,256 | ) |  | 151,193 |  |  | (2,623 | )
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commercial mortgage-backed securities |  | 120,070 |  |  | (1,721 | ) |  | 155,128 |  |  | (5,376 | ) |  | 275,198 |  |  | (7,097 | )
Residential mortgage-backed securities |  | 365,038 |  |  | (2,344 | ) |  | 288,768 |  |  | (5,538 | ) |  | 653,806 |  |  | (7,882 | )
Municipal securities |  | 22,010 |  |  | (222 | ) |  | 11,256 |  |  | (234 | ) |  | 33,266 |  |  | (456 | )
Non-agency residential mortgage-backed securities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment grade |  | 4,715 |  |  | (7 | ) |  | — |  |  | — |  |  | 4,715 |  |  | (7 | )
Corporate debt securities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment grade |  | — |  |  | — |  |  | 2,327 |  |  | (137 | ) |  | 2,327 |  |  | (137 | )
Non-investment grade |  | — |  |  | — |  |  | 9,615 |  |  | (576 | ) |  | 9,615 |  |  | (576 | )
Foreign bonds: |  |  |  |  |  |  |  |  |  |  |  |
Investment grade |  | 73,619 |  |  | (873 | ) |  | 344,298 |  |  | (15,611 | ) |  | 417,917 |  |  | (16,484 | )
Other securities |  | 31,223 |  |  | (372 | ) |  | — |  |  | — |  |  | 31,223 |  |  | (372 | )
Total available-for-sale investment securities |  | $ | 1,128,863 |  |  | $ | (10,521 | ) |  | $ | 976,729 |  |  | $ | (31,816 | ) |  | $ | 2,105,592 |  |  | $ | (42,337 | )
Held-to-maturity investment security: |  |  |  |  |  |  |  |  |  |  |  |
Non-agency commercial mortgage-backed security |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | — |
Total investment securities |  | $ | 1,128,863 |  |  | $ | (10,521 | ) |  | $ | 976,729 |  |  | $ | (31,816 | ) |  | $ | 2,105,592 |  |  | $ | (42,337 | )
|  | As of December 31, 2016
($ in thousands) |  | Less Than 12 Months |  | 12 Months or More |  | Total
| Fair Value |  | Gross Unrealized Losses |  | Fair Value |  | Gross Unrealized Losses |  | Fair Value |  | Gross Unrealized Losses
Available-for-sale investment securities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury securities |  | $ | 670,268 |  |  | $ | (9,829 | ) |  | $ | — |  |  | $ | — |  |  | $ | 670,268 |  |  | $ | (9,829 | )
U.S. government agency and U.S. government sponsored enterprise debt securities |  | 203,901 |  |  | (3,249 | ) |  | — |  |  | — |  |  | 203,901 |  |  | (3,249 | )
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities: |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commercial mortgage-backed securities |  | 202,106 |  |  | (5,452 | ) |  | 29,201 |  |  | (766 | ) |  | 231,307 |  |  | (6,218 | )
Residential mortgage-backed securities |  | 629,324 |  |  | (9,594 | ) |  | 119,603 |  |  | (1,955 | ) |  | 748,927 |  |  | (11,549 | )
Municipal securities |  | 57,655 |  |  | (1,699 | ) |  | 2,692 |  |  | (201 | ) |  | 60,347 |  |  | (1,900 | )
Non-agency residential mortgage-backed securities: |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment grade |  | 5,033 |  |  | (101 | ) |  | 6,444 |  |  | (14 | ) |  | 11,477 |  |  | (115 | )
Corporate debt securities: |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment grade |  | — |  |  | — |  |  | 71,667 |  |  | (375 | ) |  | 71,667 |  |  | (375 | )
Non-investment grade |  | — |  |  | — |  |  | 9,173 |  |  | (1,018 | ) |  | 9,173 |  |  | (1,018 | )
Foreign bonds: |  |  |  |  |  |  |  |  |  |  |  |
Investment grade |  | 363,618 |  |  | (21,327 | ) |  | 14,258 |  |  | (252 | ) |  | 377,876 |  |  | (21,579 | )
Other securities |  | 30,991 |  |  | (509 | ) |  | — |  |  | — |  |  | 30,991 |  |  | (509 | )
Total available-for-sale investment securities |  | $ | 2,162,896 |  |  | $ | (51,760 | ) |  | $ | 253,038 |  |  | $ | (4,581 | ) |  | $ | 2,415,934 |  |  | $ | (56,341 | )
Held-to-maturity investment security: |  |  |  |  |  |  |  |  |  |  |  |
Non-agency commercial mortgage-backed security |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | — |
Total investment securities |  | $ | 2,162,896 |  |  | $ | (51,760 | ) |  | $ | 253,038 |  |  | $ | (4,581 | ) |  | $ | 2,415,934 |  |  | $ | (56,341 | )
Schedule of the proceeds, gross realized gains and losses, and tax expense related to the sales of available-for-sale investment securities | The following table presents the proceeds, gross realized gains and losses, and tax expense related to the sales of available-for-sale investment securities for the three and nine months ended September 30, 2017 and 2016:
($ in thousands) |  | Three Months Ended September 30, |  | Nine Months Ended September 30,
| 2017 |  | 2016 |  | 2017 |  | 2016
Proceeds from sales |  | $ | 124,887 |  |  | $ | 143,513 |  |  | $ | 676,776 |  |  | $ | 1,008,256 |
Gross realized gains |  | $ | 1,539 |  |  | $ | 1,790 |  |  | $ | 6,733 |  |  | $ | 8,593 |
Gross realized losses |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | (125 | )
Related tax expense |  | $ | 647 |  |  | $ | 752 |  |  | $ | 2,831 |  |  | $ | 3,560 |
Schedule of maturities of investment securities | The following table presents the scheduled maturities of available-for-sale investment securities as of September 30, 2017:
($ in thousands) |  | Amortized Cost |  | Estimated Fair Value
Due within one year |  | $ | 638,257 |  |  | $ | 621,343 |
Due after one year through five years |  | 629,892 |  |  | 623,058 |
Due after five years through ten years |  | 176,117 |  |  | 172,902 |
Due after ten years |  | 1,548,497 |  |  | 1,539,473 |
Total available-for-sale investment securities |  | $ | 2,992,763 |  |  | $ | 2,956,776 |
Schedule of restricted equity securities | The following table presents the restricted equity securities as of September 30, 2017 and December 31, 2016:
($ in thousands) |  | September 30, 2017 |  | December 31, 2016
Federal Reserve Bank stock |  | $ | 56,072 |  |  | $ | 55,525 |
FHLB stock |  | 17,250 |  |  | 17,250 |
Total |  | $ | 73,322 |  |  | $ | 72,775 |

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