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Securities (Schedule of Available-for-sale Securities) (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | $ 0 | $ 143,971
Held-to-maturity investment security, at fair value | 0 | 144,593
Transfer of security, from held-to-maturity, to available-for-sale | 115,600 |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 3,070,067 | 3,385,441
Gross Unrealized Gains | 5,013 | 6,695
Gross Unrealized Losses | (58,328) | (56,341)
Available-for-sale investment securities, at fair value | 3,016,752 | 3,335,795
Marketable Securities [Abstract] |  |
Amortized Cost | 3,070,067 | 3,529,412
Gross Unrealized Gains | 5,013 | 7,317
Gross Unrealized Losses | (58,328) | (56,341)
Marketable securities, at fair Value | 3,016,752 | 3,480,388
U.S. Treasury securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 651,395 | 730,287
Gross Unrealized Gains | 0 | 21
Gross Unrealized Losses | (11,115) | (9,829)
Available-for-sale investment securities, at fair value | 640,280 | 720,479
U.S. government agency and U.S. government sponsored enterprise debt securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 206,815 | 277,891
Gross Unrealized Gains | 62 | 224
Gross Unrealized Losses | (3,485) | (3,249)
Available-for-sale investment securities, at fair value | 203,392 | 274,866
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities - Commercial mortgage-backed securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 328,348 | 272,672
Gross Unrealized Gains | 141 | 345
Gross Unrealized Losses | (9,532) | (6,218)
Available-for-sale investment securities, at fair value | 318,957 | 266,799
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities - Residential mortgage-backed securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 1,199,869 | 1,266,372
Gross Unrealized Gains | 3,964 | 3,924
Gross Unrealized Losses | (13,562) | (11,549)
Available-for-sale investment securities, at fair value | 1,190,271 | 1,258,747
Municipal securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 99,636 | 148,302
Gross Unrealized Gains | 655 | 1,252
Gross Unrealized Losses | (309) | (1,900)
Available-for-sale investment securities, at fair value | 99,982 | 147,654
Non-agency residential mortgage-backed securities | Investment grade |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 9,136 | 11,592
Gross Unrealized Gains | 3 | 0
Gross Unrealized Losses | (22) | (115)
Available-for-sale investment securities, at fair value | 9,117 | 11,477
Corporate debt securities | Investment grade |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 37,585 | 222,190
Gross Unrealized Gains | 164 | 562
Gross Unrealized Losses | (746) | (375)
Available-for-sale investment securities, at fair value | 37,003 | 222,377
Corporate debt securities | Non-investment grade |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost |  | 10,191
Gross Unrealized Gains |  | 0
Gross Unrealized Losses |  | (1,018)
Available-for-sale investment securities, at fair value |  | 9,173
Foreign bonds | Investment grade |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 505,396 | 405,443
Gross Unrealized Gains | 24 | 30
Gross Unrealized Losses | (19,012) | (21,579)
Available-for-sale investment securities, at fair value | 486,408 | 383,894
Foreign bonds | Investment grade | Multilateral development bank |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale investment securities, at fair value | 456,100 | 353,600
Other securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 31,887 | 40,501
Gross Unrealized Gains | 0 | 337
Gross Unrealized Losses | (545) | (509)
Available-for-sale investment securities, at fair value | 31,342 | 40,329
Non-agency commercial mortgage-backed security |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 0 | 143,971
Gross Unrealized Gains | 0 | 622
Gross Unrealized Losses | 0 | 0
Held-to-maturity investment security, at fair value | $ 0 | $ 144,593

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