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Investment Securities (Available For Sale Securities Unrealized Gains And Losses, By Security Type) (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Investment [Line Items]    
Amortized Cost $ 8,520,428 $ 8,715,456
Available For Sale Securities, Gross Unreailzed Gain at Period End 26,125 54,108
Available For Sale Securities, Gross Unrealized Loss at Period End (114,373) (44,122)
Fair Value 8,432,180 8,725,442
U.S. Government And Federal Agency Obligations [Member]    
Investment [Line Items]    
Amortized Cost 921,825 917,494
Available For Sale Securities, Gross Unreailzed Gain at Period End 1,617 4,096
Available For Sale Securities, Gross Unrealized Loss at Period End (11,519) (4,443)
Fair Value 911,923 917,147
Government-Sponsored Enterprise Obligations [Member]    
Investment [Line Items]    
Amortized Cost 393,331 408,266
Available For Sale Securities, Gross Unreailzed Gain at Period End 0 26
Available For Sale Securities, Gross Unrealized Loss at Period End (4,732) (1,929)
Fair Value 388,599 406,363
State And Municipal Obligations [Member]    
Investment [Line Items]    
Amortized Cost 1,417,449 1,592,707
Available For Sale Securities, Gross Unreailzed Gain at Period End 7,572 21,413
Available For Sale Securities, Gross Unrealized Loss at Period End (8,176) (2,754)
Fair Value 1,416,845 1,611,366
Agency Mortgage-Backed Securities [Member]    
Investment [Line Items]    
Amortized Cost 3,003,975 3,046,701
Available For Sale Securities, Gross Unreailzed Gain at Period End 8,074 17,956
Available For Sale Securities, Gross Unrealized Loss at Period End (57,055) (23,744)
Fair Value 2,954,994 3,040,913
Non-Agency Mortgage-Backed Securities [Member]    
Investment [Line Items]    
Amortized Cost 966,401 903,920
Available For Sale Securities, Gross Unreailzed Gain at Period End 6,530 6,710
Available For Sale Securities, Gross Unrealized Loss at Period End (12,319) (4,837)
Fair Value 960,612 905,793
Asset-Backed Securities [Member]    
Investment [Line Items]    
Amortized Cost 1,476,947 1,495,380
Available For Sale Securities, Gross Unreailzed Gain at Period End 2,323 2,657
Available For Sale Securities, Gross Unrealized Loss at Period End (13,235) (5,237)
Fair Value 1,466,035 1,492,800
Total Mortgage And Asset-Backed Securities [Member]    
Investment [Line Items]    
Amortized Cost 5,447,323 5,446,001
Available For Sale Securities, Gross Unreailzed Gain at Period End 16,927 27,323
Available For Sale Securities, Gross Unrealized Loss at Period End (82,609) (33,818)
Fair Value 5,381,641 5,439,506
Other Debt Securities [Member]    
Investment [Line Items]    
Amortized Cost 340,500 350,988
Available For Sale Securities, Gross Unreailzed Gain at Period End 9 1,250
Available For Sale Securities, Gross Unrealized Loss at Period End (7,337) (1,178)
Fair Value $ 333,172 $ 351,060