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Investment Securities Narrative (Details)
$ in Thousands
3 Months Ended
Mar. 31, 2017
USD ($)
Security
Dec. 31, 2016
USD ($)
Mar. 31, 2016
USD ($)
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  
Available-for-sale Securities, Fair Value Disclosure 2,506,017 2,559,227  
Single Bank Stock Investment, cost basis 5,100    
Single Bank Stock Investment, Fair Value $ 9,600    
Percent Ownership In An Individual Financial Institution 44.60%    
Individual Bank Stock Investment Percent to Total Portfolio 10.00%    
Equity Securities [Member]      
Schedule of Investments [Line Items]      
Amortized cost $ 11,416 12,231  
Available-for-sale Securities, Fair Value Disclosure 22,574 24,526  
Equity Securities Financial Institution [Member]      
Schedule of Investments [Line Items]      
Available-for-sale Securities, Equity Securities 21,600 23,500  
Amortized cost 10,600    
Available-for-sale Securities, Fair Value Disclosure 21,600    
Equity Securities, Other [Member]      
Schedule of Investments [Line Items]      
Available-for-sale Securities, Equity Securities 969    
Single-issuer Trust Preferred Securities [Member]      
Schedule of Investments [Line Items]      
Unrealized Loss on Securities $ 3,100    
Number of Trust Preferred Securities | Security 19    
External Credit Rating, BBB [Member] | Single-issuer Trust Preferred Securities [Member]      
Schedule of Investments [Line Items]      
Amortized cost $ 3,800    
Available-for-sale Securities, Fair Value Disclosure 2,800    
External Credit Rating, Rated Below Investment Grade [Member] | Single-issuer Trust Preferred Securities [Member]      
Schedule of Investments [Line Items]      
Amortized cost 11,600    
Available-for-sale Securities, Fair Value Disclosure $ 10,300    
Number of Trust Preferred Securities | Security 6    
Fair Value, Measurements, Recurring [Member]      
Schedule of Investments [Line Items]      
Available-for-sale Securities, Fair Value Disclosure $ 2,506,017 2,559,227  
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member]      
Schedule of Investments [Line Items]      
Available-for-sale Securities, Fair Value Disclosure 22,574 24,526  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]      
Schedule of Investments [Line Items]      
Available-for-sale Securities, Fair Value Disclosure 22,574 24,526  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member]      
Schedule of Investments [Line Items]      
Available-for-sale Securities, Fair Value Disclosure 22,574 24,526  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Securities Financial Institution [Member]      
Schedule of Investments [Line Items]      
Available-for-sale Securities, Equity Securities 21,600 23,500  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Securities, Other [Member]      
Schedule of Investments [Line Items]      
Available-for-sale Securities, Equity Securities 969 $ 1,000  
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