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Investment Securities (Available For Sale Securities Unrealized Gains And Losses, By Security Type) (Details) - USD ($)
Mar. 31, 2020
Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost $ 8,437,595,000 $ 8,435,531,000
Available For Sale Securities, Gross Unreailzed Gain at Period End 272,348,000 148,959,000
Available For Sale Securities, Gross Unrealized Loss at Period End (31,357,000) (12,864,000)
Debt Securities, Available-for-sale, Allowance for Credit Loss 0 0
Debt Securities, Available-for-sale, Fair Value 8,678,586,000 8,571,626,000
U.S. Government and Federal Agency Obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 799,200,000 827,861,000
Available For Sale Securities, Gross Unreailzed Gain at Period End 42,196,000 23,957,000
Available For Sale Securities, Gross Unrealized Loss at Period End (949,000) (42,000)
Debt Securities, Available-for-sale, Allowance for Credit Loss 0  
Debt Securities, Available-for-sale, Fair Value 840,447,000 851,776,000
Government-Sponsored Enterprise Obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 107,841,000 138,734,000
Available For Sale Securities, Gross Unreailzed Gain at Period End 5,029,000 730,000
Available For Sale Securities, Gross Unrealized Loss at Period End 0 (187,000)
Debt Securities, Available-for-sale, Allowance for Credit Loss 0  
Debt Securities, Available-for-sale, Fair Value 112,870,000 139,277,000
State and Municipal Obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 1,247,621,000 1,225,532,000
Available For Sale Securities, Gross Unreailzed Gain at Period End 38,291,000 42,427,000
Available For Sale Securities, Gross Unrealized Loss at Period End (2,844,000) (32,000)
Debt Securities, Available-for-sale, Allowance for Credit Loss 0  
Debt Securities, Available-for-sale, Fair Value 1,283,068,000 1,267,927,000
Agency Mortgage-Backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 4,011,631,000 3,893,247,000
Available For Sale Securities, Gross Unreailzed Gain at Period End 158,362,000 50,890,000
Available For Sale Securities, Gross Unrealized Loss at Period End (1,572,000) (6,173,000)
Debt Securities, Available-for-sale, Allowance for Credit Loss 0  
Debt Securities, Available-for-sale, Fair Value 4,168,421,000 3,937,964,000
Non-Agency Mortgage-Backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 708,554,000 796,451,000
Available For Sale Securities, Gross Unreailzed Gain at Period End 11,631,000 14,036,000
Available For Sale Securities, Gross Unrealized Loss at Period End (3,177,000) (705,000)
Debt Securities, Available-for-sale, Allowance for Credit Loss 0  
Debt Securities, Available-for-sale, Fair Value 717,008,000 809,782,000
Asset-Backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 1,243,235,000 1,228,151,000
Available For Sale Securities, Gross Unreailzed Gain at Period End 10,960,000 11,056,000
Available For Sale Securities, Gross Unrealized Loss at Period End (21,903,000) (5,718,000)
Debt Securities, Available-for-sale, Allowance for Credit Loss 0  
Debt Securities, Available-for-sale, Fair Value 1,232,292,000 1,233,489,000
Total Mortgage And Asset-Backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 5,963,420,000 5,917,849,000
Available For Sale Securities, Gross Unreailzed Gain at Period End 180,953,000 75,982,000
Available For Sale Securities, Gross Unrealized Loss at Period End (26,652,000) (12,596,000)
Debt Securities, Available-for-sale, Allowance for Credit Loss 0  
Debt Securities, Available-for-sale, Fair Value 6,117,721,000 5,981,235,000
Other Debt Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 319,513,000 325,555,000
Available For Sale Securities, Gross Unreailzed Gain at Period End 5,879,000 5,863,000
Available For Sale Securities, Gross Unrealized Loss at Period End (912,000) (7,000)
Debt Securities, Available-for-sale, Allowance for Credit Loss 0  
Debt Securities, Available-for-sale, Fair Value $ 324,480,000 $ 331,411,000