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Investment Securities (Available For Sale Securities Unrealized Gains And Losses, By Security Type) (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Investment [Line Items]    
Available-for-sale Debt Securities, Amortized Cost Basis $ 8,832,289 $ 8,715,456
Available For Sale Securities, Gross Unreailzed Gain at Period End 15,867 54,108
Available For Sale Securities, Gross Unrealized Loss at Period End (173,170) (44,122)
Available-for-sale Securities, Debt Securities 8,674,986 8,725,442
U.S. Government And Federal Agency Obligations [Member]    
Investment [Line Items]    
Available-for-sale Debt Securities, Amortized Cost Basis 922,431 917,494
Available For Sale Securities, Gross Unreailzed Gain at Period End 41 4,096
Available For Sale Securities, Gross Unrealized Loss at Period End (21,357) (4,443)
Available-for-sale Securities, Debt Securities 901,115 917,147
Government-Sponsored Enterprise Obligations [Member]    
Investment [Line Items]    
Available-for-sale Debt Securities, Amortized Cost Basis 228,080 408,266
Available For Sale Securities, Gross Unreailzed Gain at Period End 0 26
Available For Sale Securities, Gross Unrealized Loss at Period End (6,964) (1,929)
Available-for-sale Securities, Debt Securities 221,116 406,363
State And Municipal Obligations [Member]    
Investment [Line Items]    
Available-for-sale Debt Securities, Amortized Cost Basis 1,397,646 1,592,707
Available For Sale Securities, Gross Unreailzed Gain at Period End 3,541 21,413
Available For Sale Securities, Gross Unrealized Loss at Period End (12,289) (2,754)
Available-for-sale Securities, Debt Securities 1,388,898 1,611,366
Agency Mortgage-Backed Securities [Member]    
Investment [Line Items]    
Available-for-sale Debt Securities, Amortized Cost Basis 3,351,134 3,046,701
Available For Sale Securities, Gross Unreailzed Gain at Period End 3,875 17,956
Available For Sale Securities, Gross Unrealized Loss at Period End (88,485) (23,744)
Available-for-sale Securities, Debt Securities 3,266,524 3,040,913
Non-Agency Mortgage-Backed Securities [Member]    
Investment [Line Items]    
Available-for-sale Debt Securities, Amortized Cost Basis 1,103,357 903,920
Available For Sale Securities, Gross Unreailzed Gain at Period End 6,236 6,710
Available For Sale Securities, Gross Unrealized Loss at Period End (18,211) (4,837)
Available-for-sale Securities, Debt Securities 1,091,382 905,793
Asset-Backed Securities [Member]    
Investment [Line Items]    
Available-for-sale Debt Securities, Amortized Cost Basis 1,489,750 1,495,380
Available For Sale Securities, Gross Unreailzed Gain at Period End 2,174 2,657
Available For Sale Securities, Gross Unrealized Loss at Period End (16,499) (5,237)
Available-for-sale Securities, Debt Securities 1,475,425 1,492,800
Total Mortgage And Asset-Backed Securities [Member]    
Investment [Line Items]    
Available-for-sale Debt Securities, Amortized Cost Basis 5,944,241 5,446,001
Available For Sale Securities, Gross Unreailzed Gain at Period End 12,285 27,323
Available For Sale Securities, Gross Unrealized Loss at Period End (123,195) (33,818)
Available-for-sale Securities, Debt Securities 5,833,331 5,439,506
Other Debt Securities [Member]    
Investment [Line Items]    
Available-for-sale Debt Securities, Amortized Cost Basis 339,891 350,988
Available For Sale Securities, Gross Unreailzed Gain at Period End 0 1,250
Available For Sale Securities, Gross Unrealized Loss at Period End (9,365) (1,178)
Available-for-sale Securities, Debt Securities $ 330,526 $ 351,060