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Organization and Basis of Presentation - Reorganization, IPO and Expense Allocations (Details) - USD ($)
$ / shares in Units, $ in Millions
9 Months Ended 12 Months Ended
Aug. 09, 2016
Apr. 01, 2016
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Dec. 31, 2014
Subsidiary, Sale of Stock [Line Items]            
Threshold of U.S. non-branch assets of foreign banking organization which will require a U.S. intermediate holding company     $ 50,000.0      
BNP Paribas | First Hawaiian, Inc.            
Subsidiary, Sale of Stock [Line Items]            
Outstanding common stock owned (as a percent) 83.00%          
Reorganization Transactions on April 1, 2016 | BancWest Corporation | Bank of the West | BancWest Holding            
Subsidiary, Sale of Stock [Line Items]            
Ownership prior to transactions (as a percent)   100.00%        
Reorganization Transactions on April 1, 2016 | First Hawaiian Bank            
Subsidiary, Sale of Stock [Line Items]            
Expenses allocated to subsidiary     $ 5.8 $ 13.1 $ 18.8 $ 8.7
Initial public offering | Common Stock            
Subsidiary, Sale of Stock [Line Items]            
Shares issued 24,250,000          
Offering price (in dollars per share) $ 23.00          
Underwriters’ option | Common Stock            
Subsidiary, Sale of Stock [Line Items]            
Shares issued 3,163,043