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Subordinated Debentures and Convertible Notes - Narrative (Details)
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 07, 2018
USD ($)
$ / shares
Apr. 26, 2018
USD ($)
Jun. 30, 2019
USD ($)
grantor_trust
Dec. 31, 2018
USD ($)
shares
Jun. 30, 2018
USD ($)
Jun. 30, 2019
USD ($)
grantor_trust
shares
Sep. 30, 2018
USD ($)
shares
Jun. 30, 2018
USD ($)
Dec. 31, 2018
USD ($)
shares
May 11, 2018
USD ($)
Subordinated Borrowing [Line Items]                    
Number of wholly owned subsidiary grantor trusts | grantor_trust     9     9        
Amount of pooled trust preferred securities issued     $ 126,000,000.0     $ 126,000,000.0        
Right to defer consecutive payments of interest, maximum term           5 years        
Proceeds from convertible notes, net of issuance fees           $ 0   $ 212,920,000    
Repurchase of treasury stock       $ 50,000,000.0 $ 78,961,000 $ 100,000,000.0   78,961,000    
Common stock repurchased and recorded as treasury stock (in shares) | shares       3,436,757   0 5,565,696   9,002,453  
Other assets                    
Subordinated Borrowing [Line Items]                    
Investment in common trust securities       $ 3,900,000         $ 3,900,000  
Carrying Value of Debentures                    
Subordinated Borrowing [Line Items]                    
Unamortized Discount on Acquired Subordinated Debt     27,400,000 $ 28,000,000.0   $ 27,400,000     28,000,000.0  
Trust Preferred Securities Subject to Mandatory Redemption                    
Subordinated Borrowing [Line Items]                    
Amount of pooled trust preferred securities issued     $ 126,000,000     $ 126,000,000        
Percent included in tier one capital, maximum     25.00%     25.00%        
Excess of percent threshold included in tier two capital     25.00%     25.00%        
Convertible Notes                    
Subordinated Borrowing [Line Items]                    
Aggregate principal amount issued     $ 217,500,000   217,500,000 $ 217,500,000   $ 217,500,000   $ 200,000,000
Interest rate                   2.00%
Additional face amount issued as part of initial offering over-allotment option $ 17,500,000                  
Initial conversion rate 0.0450760   0.0450760              
Initial conversion price (in dollars per share) | $ / shares $ 22.18                  
Premium percentage to closing stock price on date of pricing of the notes 22.50%                  
Call option, percentage of principal amount in cash 100.00%                  
Repurchase or put option, percentage of principal amount in cash 100.00%                  
Amount of proceeds from convertible notes authorized for repurchase of common stock $ 100,000,000.0 $ 100,000,000.0                
Proceeds from convertible notes, net of issuance fees 213,200,000                  
Proceeds down-streamed to the Bank as equity 113,200,000                  
Repurchase of treasury stock $ 76,000,000.0           $ 24,000,000.0   $ 150,000,000.0  
Closing price of common stock (in dollars per share) | $ / shares $ 18.11                  
Amortization/ Capitalization Period 5 years         5 years     5 years  
Interest expense on convertible notes     $ 2,300,000   $ 1,200,000 $ 4,600,000        
Discount Rate | Convertible Notes                    
Subordinated Borrowing [Line Items]                    
Discount rate 0.0425