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Securities (Schedule of Available-for-sale Securities) (Details) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Available-for-sale investment securities: |  |
Amortized Cost | $ 2,890,893 | $ 3,070,067
Gross Unrealized Gains | 2,711 | 5,013
Gross Unrealized Losses | (82,188) | (58,328)
Fair Value | 2,811,416 | 3,016,752
U.S. Treasury securities |  |
Available-for-sale investment securities: |  |
Amortized Cost | 668,670 | 651,395
Gross Unrealized Gains | 93 | 0
Gross Unrealized Losses | (16,933) | (11,115)
Fair Value | 651,830 | 640,280
U.S. government agency and U.S. government sponsored enterprise debt securities |  |
Available-for-sale investment securities: |  |
Amortized Cost | 235,776 | 206,815
Gross Unrealized Gains | 175 | 62
Gross Unrealized Losses | (2,935) | (3,485)
Fair Value | 233,016 | 203,392
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities - Commercial mortgage-backed securities |  |
Available-for-sale investment securities: |  |
Amortized Cost | 352,980 | 328,348
Gross Unrealized Gains | 170 | 141
Gross Unrealized Losses | (13,316) | (9,532)
Fair Value | 339,834 | 318,957
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities - Residential mortgage-backed securities |  |
Available-for-sale investment securities: |  |
Amortized Cost | 1,008,235 | 1,199,869
Gross Unrealized Gains | 1,844 | 3,964
Gross Unrealized Losses | (20,626) | (13,562)
Fair Value | 989,453 | 1,190,271
Municipal securities |  |
Available-for-sale investment securities: |  |
Amortized Cost | 74,942 | 99,636
Gross Unrealized Gains | 251 | 655
Gross Unrealized Losses | (1,117) | (309)
Fair Value | 74,076 | 99,982
Non-agency residential mortgage-backed securities | Investment grade |  |
Available-for-sale investment securities: |  |
Amortized Cost | 8,547 | 9,136
Gross Unrealized Gains | 0 | 3
Gross Unrealized Losses | (143) | (22)
Fair Value | 8,404 | 9,117
Corporate debt securities | Investment grade |  |
Available-for-sale investment securities: |  |
Amortized Cost | 36,379 | 37,585
Gross Unrealized Gains | 178 | 164
Gross Unrealized Losses | (699) | (746)
Fair Value | 35,858 | 37,003
Foreign bonds | Investment grade |  |
Available-for-sale investment securities: |  |
Amortized Cost | 505,364 | 505,396
Gross Unrealized Gains | 0 | 24
Gross Unrealized Losses | (26,419) | (19,012)
Fair Value | 478,945 | 486,408
Foreign bonds | Investment grade | Multilateral development bank |  |
Available-for-sale investment securities: |  |
Fair Value | $ 448,500 | 456,100
Other securities |  |
Available-for-sale investment securities: |  |
Amortized Cost |  | 31,887
Gross Unrealized Gains |  | 0
Gross Unrealized Losses |  | (545)
Fair Value |  | $ 31,342

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