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Significant Accounting Policies and Basis of Presentation - Narrative (Detail)
 6 Months Ended 12 Months Ended
Jan. 01, 2018
USD ($)
Jul. 11, 2016
USD ($)
Dec. 31, 2016
Segment
Jun. 30, 2016
Segment
Dec. 31, 2018
USD ($)
derivative
Segment
CompensationPlan
Swap
swap
Dec. 31, 2017
USD ($)
derivative
Swap
Dec. 31, 2016
USD ($)
Dec. 31, 2014
Jan. 01, 2019
USD ($)
Derivative [Line Items]                  
Average reserve balances required to be maintained with federal reserve bank         $ 58,400,000 $ 164,700,000      
Held to maturity securities         $ 0 0      
Contractual payment of principal duration (days)         90 days        
Contractual terms for loans (months)         6 months        
Payments for termination to FDIC   $ 1,400,000              
Operating lease, carrying value of assets         $ 54,500,000 21,700,000      
Operating lease, accumulated depreciation         $ (4,800,000) 500,000      
Operating lease, term         4 years 7 months 4 days        
Maximum loan to value ratio         80.00%        
Delinquency period for loan charge-offs (days)         120 days        
Goodwill and other intangibles         $ 16,499,000 16,295,000      
FHLB, Federal Reserve Bank, and other restricted stock         89,685,000 105,918,000      
Federal Home Loan Bank stock         $ 67,300,000 83,700,000      
Percentage of tax position will realized or sustained upon examination         50.00%        
Number of share-based compensation plans | CompensationPlan         4        
Number of reportable segments (segment) | Segment     2 1 2        
Collaborative arrangement, expense reimbursement         $ 8,400,000 2,400,000 $ 0    
Unrealized gains (losses) on available-for-sale securities         (20,283,000) 2,520,000 $ 2,505,000    
Accounting Standards Update 2018-02                  
Derivative [Line Items]                  
Cumulative effect adjustment         0        
Accounting Standards Update 2016-01                  
Derivative [Line Items]                  
Cumulative effect adjustment         $ 0        
Net unrealized gain $ 1,041,000                
Cash Flow Hedging | Designated as Hedging Instrument                  
Derivative [Line Items]                  
Derivative notional amount           $ 550,000,000      
Number of interest rate swaps | derivative           9      
Interest Rate Swap                  
Derivative [Line Items]                  
Number of interest rate swaps | swap         3        
Interest Rate Swap | Not Designated as Hedging Instrument                  
Derivative [Line Items]                  
Derivative notional amount         $ 1,030,100,000.0 $ 800,500,000.0      
Number of interest rate swaps | Swap         98 76      
Interest Rate Swap | Cash Flow Hedging | Designated as Hedging Instrument                  
Derivative [Line Items]                  
Derivative notional amount         $ 750,000,000 $ 550,000,000      
Number of interest rate swaps | derivative         6 9      
Credit contracts | Not Designated as Hedging Instrument                  
Derivative [Line Items]                  
Derivative notional amount         $ 94,900,000.0 $ 80,500,000.0      
ESPP                  
Derivative [Line Items]                  
Purchase price under employee stock purchase plan (percent)         85.00%     85.00%  
Purchase price under employee stock purchase plan, discount (percent)         15.00%     15.00%  
Equity securities                  
Derivative [Line Items]                  
Unrealized gains (losses) on available-for-sale securities           $ 1,000,000      
Accumulated Other Comprehensive Income (Loss) | Accounting Standards Update 2018-02                  
Derivative [Line Items]                  
Cumulative effect adjustment 298,000       $ 298,000        
Accumulated Other Comprehensive Income (Loss) | Accounting Standards Update 2016-01                  
Derivative [Line Items]                  
Cumulative effect adjustment 1,041,000       1,041,000        
Retained Earnings | Accounting Standards Update 2018-02                  
Derivative [Line Items]                  
Cumulative effect adjustment         (298,000)        
Retained Earnings | Accounting Standards Update 2016-01                  
Derivative [Line Items]                  
Cumulative effect adjustment $ 1,041,000       $ (1,041,000)        
Subsequent Event | Accounting Standards Update 2016-02                  
Derivative [Line Items]                  
Operating lease, right-of-use asset                 $ 25,000,000
Operating lease, liability                 $ 25,000,000