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Investment Securities (Available For Sale Securities Unrealized Gains And Losses, By Security Type) (Details) - USD ($)
Jun. 30, 2020
Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost $ 9,961,155,000 $ 8,435,531,000
Available For Sale Securities, Gross Unreailzed Gain at Period End 369,333,000 148,959,000
Available For Sale Securities, Gross Unrealized Loss at Period End (13,061,000) (12,864,000)
Debt Securities, Available-for-sale, Allowance for Credit Loss 0 0
Debt Securities, Available-for-sale, Fair Value 10,317,427,000 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 769,463,000 827,861,000
Available For Sale Securities, Gross Unreailzed Gain at Period End 57,031,000 23,957,000
Available For Sale Securities, Gross Unrealized Loss at Period End 0 (42,000)
Debt Securities, Available-for-sale, Allowance for Credit Loss 0  
Debt Securities, Available-for-sale, Fair Value 826,494,000 851,776,000
Government-Sponsored Enterprise Obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 106,019,000 138,734,000
Available For Sale Securities, Gross Unreailzed Gain at Period End 4,096,000 730,000
Available For Sale Securities, Gross Unrealized Loss at Period End (232,000) (187,000)
Debt Securities, Available-for-sale, Allowance for Credit Loss 0  
Debt Securities, Available-for-sale, Fair Value 109,883,000 139,277,000
State and Municipal Obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 1,385,113,000 1,225,532,000
Available For Sale Securities, Gross Unreailzed Gain at Period End 64,073,000 42,427,000
Available For Sale Securities, Gross Unrealized Loss at Period End (234,000) (32,000)
Debt Securities, Available-for-sale, Allowance for Credit Loss 0  
Debt Securities, Available-for-sale, Fair Value 1,448,952,000 1,267,927,000
Agency Mortgage-Backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 5,205,895,000 3,893,247,000
Available For Sale Securities, Gross Unreailzed Gain at Period End 176,441,000 50,890,000
Available For Sale Securities, Gross Unrealized Loss at Period End (149,000) (6,173,000)
Debt Securities, Available-for-sale, Allowance for Credit Loss 0  
Debt Securities, Available-for-sale, Fair Value 5,382,187,000 3,937,964,000
Non-Agency Mortgage-Backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 581,257,000 796,451,000
Available For Sale Securities, Gross Unreailzed Gain at Period End 19,013,000 14,036,000
Available For Sale Securities, Gross Unrealized Loss at Period End (74,000) (705,000)
Debt Securities, Available-for-sale, Allowance for Credit Loss 0  
Debt Securities, Available-for-sale, Fair Value 600,196,000 809,782,000
Asset-Backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 1,431,854,000 1,228,151,000
Available For Sale Securities, Gross Unreailzed Gain at Period End 28,743,000 11,056,000
Available For Sale Securities, Gross Unrealized Loss at Period End (12,372,000) (5,718,000)
Debt Securities, Available-for-sale, Allowance for Credit Loss 0  
Debt Securities, Available-for-sale, Fair Value 1,448,225,000 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 7,219,006,000 5,917,849,000
Available For Sale Securities, Gross Unreailzed Gain at Period End 224,197,000 75,982,000
Available For Sale Securities, Gross Unrealized Loss at Period End (12,595,000) (12,596,000)
Debt Securities, Available-for-sale, Allowance for Credit Loss 0  
Debt Securities, Available-for-sale, Fair Value 7,430,608,000 5,981,235,000
Other Debt Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 481,554,000 325,555,000
Available For Sale Securities, Gross Unreailzed Gain at Period End 19,936,000 5,863,000
Available For Sale Securities, Gross Unrealized Loss at Period End 0 (7,000)
Debt Securities, Available-for-sale, Allowance for Credit Loss 0  
Debt Securities, Available-for-sale, Fair Value $ 501,490,000 $ 331,411,000