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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