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Investment Securities Schedule of Amortized Cost and Fair Values of Investment Securities (Details) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Mar. 31, 2016
Schedule of Investments [Line Items]      
Available-for-sale Securities Pledged as Collateral $ 1,800,000 $ 1,800,000  
Amortized Cost 2,535,576 2,594,255  
Gross Unrealized Gains 23,568 24,807  
Gross Unrealized Losses (53,127) (59,835)  
Estimated Fair Value 2,506,017 2,559,227  
U.S. Government-Sponsored Agency Securities [Member]      
Schedule of Investments [Line Items]      
Amortized Cost 5,959 132  
Gross Unrealized Gains 37 2  
Gross Unrealized Losses 0 0  
Estimated Fair Value 5,996 134  
State and Municipal Securities [Member]      
Schedule of Investments [Line Items]      
Amortized Cost 404,125 405,274  
Gross Unrealized Gains 2,352 2,043  
Gross Unrealized Losses (13,853) (15,676)  
Estimated Fair Value 392,624 391,641  
Corporate Debt Securities [Member]      
Schedule of Investments [Line Items]      
Amortized Cost 109,595 112,016  
Gross Unrealized Gains 2,055 1,978  
Gross Unrealized Losses (3,773) (4,585)  
Estimated Fair Value 107,877 109,409  
Collateralized Mortgage Obligations [Member]      
Schedule of Investments [Line Items]      
Amortized Cost 575,596 604,095  
Gross Unrealized Gains 1,811 1,943  
Gross Unrealized Losses (11,521) (12,178)  
Estimated Fair Value 565,886 593,860  
Mortgage-Backed Securities [Member]      
Schedule of Investments [Line Items]      
Amortized Cost 1,321,573 1,353,292  
Gross Unrealized Gains 6,155 6,546  
Gross Unrealized Losses (14,107) (17,437)  
Estimated Fair Value 1,313,621 1,342,401  
Auction Rate Securities [Member]      
Schedule of Investments [Line Items]      
Amortized Cost 107,312 107,215  
Gross Unrealized Gains 0 0  
Gross Unrealized Losses (9,873) (9,959)  
Estimated Fair Value 97,439 97,256  
Debt Securities [Member]      
Schedule of Investments [Line Items]      
Amortized Cost 2,524,160 2,582,024  
Gross Unrealized Gains 12,410 12,512  
Gross Unrealized Losses (53,127) (59,835)  
Estimated Fair Value 2,483,443 2,534,701  
Equity Securities [Member]      
Schedule of Investments [Line Items]      
Amortized Cost 11,416 12,231  
Gross Unrealized Gains 11,158 12,295  
Gross Unrealized Losses 0 0  
Estimated Fair Value 22,574 $ 24,526  
Pooled Trust Preferred Securities [Member]      
Schedule of Investments [Line Items]      
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held $ 11,500   $ 11,500